Warning issued to Sonny Bill Williams after the recent return of Ben Te'o to NRL

Brisbane coach Anthony Seibold has warned Sonny Bill Williams the NRL has changed in his absence after watching Ben Te'o struggle to get up to speed. Williams was bound for Sydney on Wednesday, leaving his base in Manchester and ending a rare period in 'holiday mode' for the dual-code star.

The 34-year-old had planned a European getaway once Toronto pulled out of the Super League, but he has now 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 18 Test games for England as well as two more with the British and Irish Lions when 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.

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

Despite the warning, Williams insisted he is ready for the challenge of returning to the NRL code that made him. "It's the challenge that lights a fire inside of you," he 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 (the club chairman) and Trent (the coach). 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 he isn't expected to play before round 17.