VIDEO: SBW retains unbeaten run
Williams recorded his seventh boxing career victory with a unanimous points win against Welliver.
The New Zealander dominated an out of shape Welliver for most of the fight. He now boasts a 7-0 career win/loss record in the boxing ring.
Welliver, who has sparred with the likes of Wladimir Klitschko, Mike Tyson and David Tua in a 14-year professional career, was no match for Williams in the All Black's return to the ring for the first time since surviving a 10 round bout with South Africa's Francois Botha in Brisbane two years ago.
The 135kg American afterwards said Williams could be the real deal in boxing.
"This guy is going to be the greatest thing coming out of Australia (sic), he is a great rugby player and he is a great boxer," Welliver told the Sydney Morning Herald. "This kid can be a contender. He isn't even a full-time boxer and he beat my a**e today (Saturday)."
In one of the undercards, All Blacks and Chiefs loose forward Liam Messam made a successful start to his professional boxing career by claiming a unanimous decision over winless but rugged Australian heavyweight Rhys Sullivan, the Stuff.co.nz website reports.
Messam, who had previously fought twice in the annual Fight for Life charity promotion against Wendell Sailor and Paul Gallen, completed the step-up to his first serious bout in style as he dominated the little-known Sullivan, who slumped to his fourth straight defeat.
Sonny Bill Williams and Chauncy Welliver(Getty Images)