The 28-year-old is to play for rugby league's Sydney Roosters, the club he helped to Australia's National Rugby League (NRL) title this year, in 2014 before rejoining the Waikato Chiefs on a two-year contract.
"We made no secret we were keen to get Sonny back in a Chiefs jersey," Waikato coach Dave Rennie said in a statement. "He's not only a quality player but adds huge value to our environment off the field."
The player spent two years in New Zealand rugby, helping the All Blacks to the 2011 World Cup and the Chiefs to their inaugural Super Rugby title in 2012.
The Hamilton-based franchise retained their crown in 2013 without Williams who had left New Zealand to take up a short-term contract in Japan before switching codes back to rugby league.
Williams has put professional boxing aspirations on hold and has made no secret of wanting to cap his career with an Olympic gold medal in rugby sevens.
The New Zealand Rugby Union (NZRU) is pleased to have secured Williams, who has signed a succession of one-year deals since walking out on a multi-year deal with the NRL's Canterbury Bulldogs to play union in 2008, on a two-year contract so he can push for sevens selection in Rio.
"Sonny showed in his two years with New Zealand rugby that he is an outstanding talent and was a real asset to our game," said NZRU chief executive Steve Tew.
"We think it is great he has made his aspirations clear and if he plays well enough to be considered for selection for those teams, then that is fantastic."
(Reporting by Greg Stutchbury, editing by Tony Jimenez)
