Of course, most of us don't have to walk the corridors of Parliament House on a regular basis, and few of us need to concern ourselves with the intellectual voyages of political life. But this year, every Australian of voting age was asked to make a personal journey in the service of Prime Minister Malcolm Turnbull's government. More than 12 million of us - almost four-fifths of Australia's eligible voters - walked to post boxes, pulled up in no-parking zones, and queued in post offices to send our answers to Canberra on the question of whether same-sex couples should be allowed to marry. And close to eight million of us, 62 per cent, said yes.

What few realised when we made these journeys was that at the end of them all was not a prime minister or political party poised to turn our wishes into law. Instead, there was just one dark-haired man, wearing a neatly pressed shirt and tie, sitting in an office along one of those wide white corridors. This Christmas, every Australian is free to marry the person they love - for the first time ever - because of him.

Dean Smith describes himself as the invisible man. "I'm that person in the room nobody notices," he says evenly. "People don't even know I'm there." This is not true. I met Smith through friends more than a decade ago, in our early 30s, and though we didn't know each other well, I remembered him. He wore perfect collared shirts, was a devout Anglican, and was gay. He was also a Liberal Party member, the first member of his working-class family to go to university (to study politics) and a proud monarchist. As if all this wasn't unusual enough, he was earnestly, unashamedly enthusiastic about politics. He had no cynicism at all. A mutual friend remembers him leaning over in a taxi in Sydney's Kings Cross, in the middle of a big night out, and confessing fervently: "I am a passionate federalist!"

In those days, Smith was working in Sydney for insurance firm IAG, after periods on the staff of WA premier Richard Court, prime minister John Howard, and then-minister for aged care Bronwyn Bishop. But after he left Sydney in 2007, I completely lost touch with him. I had no idea he'd become a senator for Western Australia in 2012. Then, on November 16 this year, the day after the postal survey result was announced, I saw a photo of a small man in an immaculate suit and tie, green eyes crinkling as he smiled. "Senator Dean Smith," ran the caption, "whose private member's bill to legalise same-sex marriage will go before the Senate tomorrow." Dean Smith? I thought. That Dean Smith?

A week later, when I confess this story over dinner at a Chinese restaurant in Canberra, he smiles. "See?" he says in his quiet voice. "Invisible man."

Senator Dean Smith's office in Parliament House was the brains trust of the same-sex marriage legislation. Photo: Alex Ellinghausen

???It's Monday morning in the final week of November, and Smith's snappily titled Marriage Amendment (Definition and Religious Freedoms) Bill 2017 is set to be debated in the Senate. Sitting in his office at 7am, the man himself, now 48, looks surprisingly similar to my memory of him 15 years ago: short and compact (though with a little anno domini expansion), with a cherubic face and another perfect collared shirt. A large, signed photograph of Queen Elizabeth II hangs behind his desk, and a small, dark crucifix sits beside several endearingly unprofessional family photos. Through a connecting door, his two staff, Kyle Regan and Kelli Orrell, chat as they work.

Because Smith's is a private member's bill, his office is responsible for all the administrative details of debating and voting on it. Since only 15 private bills have actually passed into Australian law since 1901, it's fair to say nobody's feeling entirely up to speed with this process. "Normally, when a minister introduces legislation, he's got, like, 20 staff," explains electorate officer Regan, who is in his 20s, with Clark Kent glasses and a subversive Lincolnshire accent. "Actually, a minister's got a whole department: thousands of people. Plus advisors. Dean's got, well, Kell and me."

Kelli Orrell, Smith's parliamentary research officer, is a small, dark-haired woman in her 40s, with a gentle face and a do-not-mess-with-me air. She has already taken 17 calls about senators who want to speak in the chamber on the bill, and it's not yet 9am. "It's ridiculous," she says.

"Everyone wants to go at the end of the list." Speaking later is seen to be better strategically. She consults her sheet of paper. "If [Sam] Dastyari wants to speak, he could go here. And [Barry] O'Sullivan wants to be here." She looks up as the phone rings. "That'll be another one."

"It could be worse," says Regan. "It could be that crazy guy." Since the mid-November postal survey result, people have been calling the office every day to scream abuse. Smith comes through. "How's it going?" he asks.

"Everybody wants to go at the end!" says Orrell.

"If everybody goes at the end, there is no end. Tell them to go through me," Smith offers.

"No, I'll deal with them," says Orrell, squaring her shoulders.

"Remember Kell, you're the point of the sword," says Regan.

I ask Smith how he's feeling. "A little anxious," he admits. "I've got to remember that it's not personal. The measure of success is not whether 100 per cent of my bill goes through. It's whether the bill most accurately reflects the views of the 76 senators."

In front of him, Regan picks up the ringing phone, listens for a minute, and hangs up. "Crazy guy," he nods. Smith smiles. "Not personal."

Smith's staff members, Kelli Orrell and Kyle Regan, at their office at Parliament House. Photo: Alex Ellinghausen

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Now that the deed is done, it's easy to say the zeitgeist was in favour of marriage equality, that the political tide had turned thanks to the sweeping yes vote - that any bill would have served the purpose. But of course, as usual in politics, things were more complicated than that. The final defeat of the government push in the Senate for a plebiscite on same-sex marriage created political turmoil for the Coalition: insistence by the conservative right that the issue was off the table, pushback from moderates who weren't prepared to let it go.

Earlier this year, five "rebel" Liberal MPs - four of them gay (Dean Smith, Trent Zimmerman, Trevor Evans and Tim Wilson), plus the resolutely non-gay Warren Entsch - threatened to cross the floor if same-sex marriage wasn't addressed by a free vote in Parliament. At a Liberal party room meeting in August, however, their proposal was roundly defeated. Instead, the postal survey was announced - with the tacit understanding that, if the result was yes, a private member's bill could be brought. So, on November 15, the day of the survey result, Smith introduced his bill.

Why was it Smith who took the lead, and not someone better known or more powerful? Well, earlier in the year, he was part of a Senate select committee that investigated same-sex marriage and produced a rare consensus report. Smith was the only person who took this report and - in his own time and without encouragement from the broader Liberal Party - went to the trouble of drafting a bill, taking care to ensure it was moderate enough to attract cross-party support. Through this process, it became widely acknowledged that Smith had carriage of the issue; that he cared enough to stick with it. It suited senior Coalition members, too, to let a lowly backbencher take the heat from the conservative right: Turnbull, for instance, maintained a hands-off position, barely speaking to Smith between the announcement of the postal survey and the debate of his bill in Parliament.

There's one other reason Smith has become the unlikely hero in the same-sex-marriage story. Not political, but personal: the relationships he built with perhaps the two most significant cross-party senators on the issue, the Opposition's leader in the Senate, Penny Wong, and Greens Senator Janet Rice.

"With Dean there were no political machinations, which is incredibly rare," says Rice. "We were able to be open and honest and upfront. You can't do that with many politicians because you just can't trust them. But I feel I could ask Dean's personal opinion on any issue, and I would trust it."

Wong echoes these words. "He has been utterly trustworthy. Through a very long process, only some of which has been public, he has always behaved with dignity and absolute integrity." Labor sources say that over the past year Smith had become one of the "very, very few" people Wong would deal with on marriage equality.

Such confidence is all the more remarkable because, on the face of it, Smith has almost no "successful politician" qualities. Parliament is a place that rewards big personalities: the pompous self-satisfaction of Attorney-General George Brandis; the super-wealthy assurance of Malcolm Turnbull (rather less evident in recent times, admittedly); even the macho posturing of former prime minister Tony Abbott. Smith is shy and self-critical. He describes himself as a "trundler", working away in obscurity. But perhaps this is the point. As Rice puts it: "If it had been Brandis - who has been a champion, in his way, for equality - leading the charge, we would not have got there."

Senator Dean Smith and Attorney-General George Brandis during the Senate debate of Smith's bill. Photo: Alex Ellinghausen

No one should underestimate the difficulties of getting there. In 2004, then prime minister Howard pushed through changes to the Marriage Act specifically stating that same-sex unions "must not be recognised as marriage in Australia". Twenty-two attempts at legislation since have tried to reverse these changes. All have failed. So how does a gay, Christian, monarchist, working-class, Liberal backbencher become the kind of man who can make the 23rd attempt stick?

Dean Smith grew up in a close, loving family in Nollamara, Perth. Mum Judy stayed at home with Dean and his two younger siblings, while his dad, Alan, who fought in Vietnam, spent more than three decades as a police officer. As a child, Smith was deeply affected by his father's anger over Labor prime minister Gough Whitlam's attitude towards Vietnam veterans, and later by the scandals of the WA Inc era under Labor premier Brian Burke.

Instead, Smith was drawn to classical conservative values: the belief that institutions and traditions form the bedrock of a well-ordered civic society. His great political hero is George VI, he of The King's Speech fame. He joined the Liberal Party at 17, on his first day at the University of Western Australia. It took him three months to work up the courage to attend a meeting, but once he got there he loved it, beavering away as a Young Liberal, winning awards like Best Branch President (1991), of which he's still proud.

Bound up in his love for conservatism is what he calls his "political birthmark": his belief in integrity. "I think you must act on your principles, regardless of consequence," he says. "I realise that isn't a very good career move, or a very effective way of doing things sometimes, because it's not transactional. But that is who I am." He was the only Liberal MP to publicly oppose the same-sex marriage plebiscite and refuse to vote for it, and the Coalition's most vocal critic of the postal survey. "Parliament's essential function is to make decisions on behalf of the Australians who have elected them," he says. "It's like saying that an institution which has served the nation well for 115 years is suddenly, on one issue alone, not up to the job."

In his personal life, Smith describes himself as "single, but aspirational". He had no idea he was gay until after university. He'd never had a gay experience. "We never discussed anything like that in my family, so I had absolutely no reference points.

"There was a boy at high school a year ahead of me, but the way I understood it was that I admired people, I didn't find them attractive." He smiles. "I have learnt slowly over life, you can't be there just for the conversation."

He came out at 24, having had an epiphany while listening to, of all people, Rod Stewart on his car radio. "I know! It just hit me. It was The Killing of Georgie [a song about the death of a young gay man]." He told his parents after a family dinner soon afterwards. "They had asked me if everything was okay. Eventually I remember Dad saying, 'Are you homosexual?' which made me laugh, because it sounded so clinical. But I said, 'Yes. I didn't want to hurt you or make you worried.' And they never wavered. They showed me unconditional love."

Smith was the first openly gay Liberal senator in Australian history. But his journey to same-sex marriage has been slow. In 2012, he voted against it in the Senate, based on his conservative belief in marriage as an institution "laden with cultural meaning and values" that didn't need to include same-sex marriage. In 2013, he criticised then governor-general Quentin Bryce for using her position to support same-sex marriage in the Boyer Lectures.

Over time, however, he thought more about the issue. "I began to think, 'Here is this cherished institution: how are you destroying or diminishing it by allowing more people to embrace it?' " He raises his hands. "That actually sustains and protects it."

The final piece of the puzzle fell into place in 2015, during a flight between Perth and Albany, in Western Australia's far south-west. "I was thinking about Tori Johnson, who lost his life in [Sydney's December 2014] Lindt cafe siege," he recalls. "He had a male partner of 14 years. Of course I have no idea whether he wanted to get married, but I realised that if he had, we had denied him that right. And that seemed wrong."

And so Smith became an unlikely campaigner for same-sex marriage: initially for others, but perhaps, increasingly hopefully, for himself. "I've been single since entering Parliament," he says. "I never thought marriage was for me. But having seen the excitement and joy in others over the past few weeks, I could be convinced."

Back in 2015, however, such joy seemed a long way away. There is a quote usually attributed to Edmund Burke, the famous 18th-century conservative political theorist whom Smith has always admired: "The only thing necessary for the triumph of evil is for good men to do nothing." When the moment of marriage-equality truth arrived, Smith was not prepared to do nothing.

Within Parliament House's countless kilometres of corridors, the Senate chamber is close to Smith's office: a few minutes' slog down one very long corridor. Smith's seat in the chamber is surrounded by those of marriage-equality opponents, because they are, by and large, members of his own party: Liberals Eric Abetz, David Fawcett, Zed Seselja, and Nationals Matthew Canavan and Barry O'Sullivan. All are religious; all are self-proclaimed defenders of traditional values.

Opposite are the bulk of his supporters: mostly the serried ranks of Labor, including Senator Wong, who has a long-term female partner, Sophie Allouache, and two daughters. There's also Louise Pratt, an openly gay woman and mother of a little boy she co-parents with her former partner (a trans man) and two gay friends, one of whom is the biological father. In a little wedge in the corner sit the Greens leader Richard Di Natale in his rainbow sneakers and Janet Rice, whose partner, Penny, is a trans woman. The pair married as husband and wife more than three decades ago, and have two grown children. They've lived in legal limbo since Penny transitioned. Down and across the aisle is Justice Party Senator Derryn Hinch, resplendent on this late November day in a rainbow scarf.

Justice Party Senator Derryn Hinch speaks during the debate. Photo: AAP

A fortnight previously, Hinch was one of eight co-sponsors of Smith's bill - along with Wong, Pratt, Rice, Di Natale, former Nick Xenophon Team Senator Skye Kakoschke-Moore (who has since resigned over citizenship issues), and Liberals Jane Hume and Linda Reynolds. That such a politically diverse group came together was thanks to Smith's bill's compromise position. It does not allow for conscientious objection, which means the infamous "bakers and florists" cannot refuse service to same-sex couples, which pleased the Greens and Labor. And it does allow ministers of religion and religious organisations - plus a new group of religious celebrants - to refuse to marry same-sex couples (or rent them a church hall, for instance), if doing so is against their beliefs. This, in turn, mollified the Coalition.

"It's not a bill that's going to appeal to either extreme," Smith explains to me. "It's a sensible, centrist, compromise bill."

Wong recalls signing Smith's bill on November 14, the day before it was introduced in the Senate. She had never co-sponsored a marriage equality bill before. "I didn't want to be part of a bill that was a political stunt. I took a view some years ago that I would only sponsor a bill that had a good chance of passing." She pauses. "Success has many fathers, and mothers. And there are a few people now who are wanting to be parents. But there are some people who were critical. Dean was one of them."

"Lovely Dean," recalls Senator Hinch's personal assistant Annette Philpott. "He kept popping in on the 14th, asking for Derryn, and I kept saying, 'Is it something I can help you with?' And he was like, 'Oh no, that's fine, I'll come back.' And then of course it was the bill! Would Derryn sign it? And Derryn was like, 'Of course I bloody will, mate!' "

ALP Senator Penny Wong celebrates with colleagues after the Senate passes the bill. Photo: Alex Ellinghausen

"That's right," says the ever-ebullient Hinch. "Last year when he was under so much pressure - huge - from the right wing of his party, to delay, delay, drop the whole thing, I said, 'If you want to go it alone, and you need a co-sponsor, I'll do it. I was thrilled to do it.' "

Hinch's office is right next door to Smith's, and the pair see each other often. "There were times when he was almost shaking from the stress of it," Hinch explains. "He's very self-effacing. He doesn't come across as a guy who's going to be a braveheart, or a guy with a cudgel. But he's a guy whose time has come. This is his issue, and he's holding firm." He sits back in his chair. "That takes guts."

Back in the chamber in the late afternoon of November 28, the second day of Senate debate, proposed amendments to Smith's bill are voted down one by one. Apart from some minor technical points, it hasn't had a single substantive change made to it. Even Smith could not have foreseen this outcome. For the past six months, on this issue at least, he's existed in a strange world of unlikely external allies - Wong, Rice et al - and internal attacks. His opponents have used the media to savage him; they've made thinly veiled suggestions that his stance is career-limiting, and even criticised him to his face.

After his crushing party room defeat in August, for instance, things were so bad that one Liberal colleague actually sent him a condolence note. "Dear Dean. Just a short note after party room to say how much I respect you for putting the case in the way you did ??? I know what it is to be in the minority in the party room. It is no fun. I especially think some of the attacks and threats against you have been appalling and un-Australian." There is a sense during the Senate debate, however, of the tide turning. Towards the end of the second day of debate, George Brandis, leader of the government in the Senate, delivers a barnstorming speech.

"The bill we are debating tonight is the work of many hands," he proclaims. "But it is our colleague Dean Smith who was the author of the final version. Dean has, through a long and politically difficult process, displayed abundant tenacity and conspicuous moral courage. I was privileged to have been Dean's confidante at critical times in the last several weeks, and I know better than most the burdens of stress, of loneliness and of hurt he endured to make Australia a better place for countless others.

"Whatever else he may achieve in what has already been a significant political career, Dean Smith will always be remembered for this."

Senators Louise Pratt, Janet Rice, Dean Smith, Skye Kakoschke-Moore, Penny Wong and Derryn Hinch celebrate the passing of the marriage equality bill in the Senate. Photo: Andrew Meares

The next day, Smith and his staff are in the office by 7am (again). It seems increasingly likely that his bill will be voted on today. Regan and Orrell are both dressed in pink. Orrell is printing yet another list; Regan is on the phone. "Yes," he's saying. "We were pretty busy with the speakers' lists - but it looks like the vote will be today. Kelli's nearly had a heart attack a couple of times. She's lucky I'm a first-aid officer." He pauses, listening. "Yes, it's true. I might have to save her life."

"In that case, I think I'd rather not survive," says Orrell from the printer.

Smith comes out of his office. He didn't sleep much last night. "I woke up feeling as if I'd had a few drinks, which I certainly hadn't." He had an emergency gall bladder operation a fortnight ago, leaving hospital after only three days to appear on TV to talk about his bill. He's barely eating - yesterday his total food consumption consisted of one apple. He looks like a man who needs to sit down. Instead he stands, holding his right side with one hand, his double espresso with the other. "Nearly there," he says.

When the day begins in the Senate chamber, nobody wants to sit down, either. Liberal Senator Ian Macdonald goes berserk about Hinch's rainbow scarf being in contravention of Senate dress code; Greens Senator Peter Whish-Wilson claims that "if Senator Hinch has to remove his scarf, Senator [Sarah] Hanson-Young will have to remove her dress". Arch-conservative Liberal Senator Eric Abetz sympathises with the "many gay people" who voted no; Senator Louise Pratt heaves a theatrical sigh. Through it all, Smith sits quietly, leafing through his notes.

Above the senators, the public galleries are filling with spectators: a baby with his father, waving his tiny arms; an old man with a hat and a long grey beard. Wong arrives wearing black and white (no rainbows), and moves the third reading. One of the most touching final speeches comes from Linda Reynolds, one of Smith's rare Liberal supporters. She stands, turns to Smith, and says simply, "I've never been prouder to stand up in this place than I have been to stand with you, Dean."

Smith, who has been organising his papers into a neat rectangle, bows his head. When he stands to speak, he chokes over the names of these internal allies, and has to pause. Watching him, both Reynolds and fellow Liberal Jane Hume are in tears. "There have been times when it has been tough to not be part of the majority of my party on this issue," he says finally. "I had to find my place where my conscience and my duty could be reconciled. So I say to all in this chamber: be kind to those who, in following their conscience, chose a different path. They have my respect, and I ask you to give them yours."

When he sits down, Wong looks over. "Well done," she mouths.

And then Smith's bill is voted on. There are 43 yes votes, 12 no's, and four official abstentions. The gallery - and the Senate chamber - erupt. People cheer, laugh and embrace. The old man with the beard weeps into his hat.

A fortnight later, the House of Representatives will follow the Senate, and pass the bill with no amendments. The Marriage Amendment (Definition and Religious Freedoms) Bill 2017 passes into law at midnight on December 8. But before that, here on the floor of an institution he deeply loves, Smith is engulfed. A great crowd of senators surround him, hugging him, holding him, shaking his hand - so many that for a moment he's invisible. Almost.