Williams was bound for Sydney on Wednesday night, leaving his base in Manchester and ending a rare period in "holiday mode" for the dual-code star.

The 34-year-old had instead planned a European getaway once Toronto pulled out of the Super League, but made a snap decision to return to the NRL.

It took just a few calls from Trent Robinson and Nick Politis to have him back at the injury-plagued Roosters and part of their hunt for an elusive three-peat.

But now Seibold warns the toughest part awaits.

Like Williams, Te'o also switched to rugby union at the end of 2014, after winning that year's premiership with Souths.

He too had success in the 15-man game, playing 16 Tests for England as well as with the British and Irish Lions just as Williams was playing as an All Blacks star.

Te'o made his return for Brisbane last month and Seibold said the 33-year-old had found it physically tougher than expected.

"It's been challenging for Ben - it's been probably a little harder than what he thought it was going to be," Seibold said on Wednesday.

"The ball-in-play time is significantly larger in rugby league compared to international and club rugby.

"Ben has felt the game physically has advanced immensely since he last played and from a rugby union players' perspective it's been really challenging."

Seibold said to Williams' advantage he played five games with the Wolfpack this year before the Super League shutdown.

"He's played some games this year while Ben came in cold; we couldn't even play him in the Queensland Cup because of that competition being suspended," the coach said.

"Ben's had to learn again on the run, after six years, and it's been challenging for him."

But Williams is ready for the challenge of returning to the code that made him.

"It's the challenge that lights a fire inside of you," Williams told the Nine Network.

"You're a long time retired.

"Everyone knows it's no secret that I'm pretty close with the lads at the Roosters, Nick (chairman Nick Politis) and Trent (coach Trent Robinson).

"I stay in contact with them now and then.

"Trent just gave me a call and inquired about what was happening over here and Nick hit me up and asked if I had a couple of months in me to come back.

"To be honest, I hadn't trained for a bit and I'd been in holiday mode."

Williams and his family will spend two weeks in quarantine upon arrival in Australia and isn't expected to play before round 17.

Australian Associated Press