September 17 is now Australia's most common birthday, according to the latest figures from the Australian Bureau of Statistics, thanks to the magic of festive timing. April 8 comes in second, with the help of some winter warming.

"These trends are similar in New Zealand, England and Wales, and the United States," said Beidar Cho, the bureau's director of demography.

"More babies are likely to be conceived around the Christmas and New Year holidays, [while] fewer babies are born on public holidays - possibly a result of doctors scheduling deliveries on non-public holidays."

Wednesday's release tells a well-worn story: disadvantaged communities are having more children at a younger age, wealthier areas are having have fewer kids, older.

Fertility rates between the centre of Australia's two largest cities almost double once you reach parts of their outer fringes - from 0.9 babies per person in central Melbourne or Sydney to 2 in Casey or Blacktown.

The median age of mothers is 33 in North Sydney, Hornsby and Melbourne's inner-east, but only 28 in western NSW and north-west Victoria.

Teenagers are more likely to give birth in the bureau's designated "outback" regions - across the Northern Territory, Western Australia, Queensland and South Australia - than anywhere else.

New England has the highest rate of teen births in NSW, followed by the central-west and the Hunter Valley along with Shepparton and Bendigo in Victoria.

Australia-wide 311,100 births were registered in 2016, up from 305,400 in 2015, while the country as a whole recorded exactly the same fertility rate as it did in 2015 of 1.79.

At the same time, the number of babies being born in NSW dropped by 4 per cent, while Victoria surged by 12 per cent from 73,600 to 82,900.

If the trend continues, it will buckle maternity wards in Melbourne, as hospitals scramble to provide services in new suburbs being built on the back of an immigration boom.

Likewise a 4 per cent drop in the number of babies being born in NSW from 100,000 to 96,000 on the back of a 9 per cent drop in 2014 has stirred fears of a baby drought.

But a clue to the discrepancy between the states lies in the bureau's own explanatory notes.

Birth registrations have been lagging in NSW. A new data processing system meant there was a drop in the number of babies being registered in time by the Office of Births Deaths and Marriages in 2014-15.

It is a trend that was first picked up on by .id demographer Simone Alexander, who warned it could have serious implications for public policy if it was not addressed.

"State and local government also use this data in their planning work," she said in 2015.

"Imagine a scenario where the lower number of births is used as evidence to cut funds to child health or education programs?"

A baby boom in Melbourne's western growth corridor is showing no signs of slowing down with a record number of births at Werribee Mercy Hospital in 2016.

There were 3748 babies, including 50 sets of twins, born at the hospital last year.

That's up from 3411 bubs in 2015, and 3014 births the year before.

Caroline Machete, 28, who gave birth to her second child Kloe at the hospital on Monday, said she wasn't surprised by the surge in births in Victoria.

"Holding your baby or hearing them cry for the first time is one of the best feelings in the world," she said. "It's just one of those unexplainable feelings, you feel whole and like nothing else in the world matters. From the moment I saw her, I was in love."