New Zealand's Sonny Bill Williams has confirmed his retirement from both rugby codes and said he was looking to restart his boxing career.

Williams fitted in seven fights between 2009 and 2015, winning each contest.

"I'm keen to get back into the ring, I've got a couple of years," he told the Nine Network.

"I spoke to the wife first and... like usual, she supported me.

"I want to know how I will go in six months instead of six weeks."

Williams switched between league and union during his 17-year career, winning NRL titles with the Canterbury Bulldogs and Sydney Roosters and the 2012 Super Rugby title with the Chiefs.

Aside from his World Cup victories with the All Blacks, he was also involved in the side that drew the 2017 series against the touring British and Irish Lions.

Williams played his last game of rugby league with the Roosters in October in the semi-final loss to the Canberra Raiders in the 2020 NRL playoffs. He was contracted to play in the 2021 season with Toronto Wolfpack, but the deal fell through after the Canadian team was denied readmission to the largely England-based Super League.

Williams added that issues with his knee meant he could no longer continue to meet the demands of rugby.

"A massive thank you to the public and fans for all the support over years," he tweeted on Thursday.

"To my many team-mates, thanks for the lessons and help along the way. It wasn't a perfect journey, but the lessons have helped shape me into the man I'm proud to be off the field today."