The 26-year-old dual international told reporters in Hamilton on Monday he will return to the NRL next year but refused to nominate which club he’ll be playing for.
Williams, who is believed to be joining the Sydney Roosters, says he had a “handshake agreement” which was put in place some years ago but he hasn’t signed anything yet.
After this year’s Super Rugby season with the Chiefs ends he will join the Panasonic Wild Knights in Japan for a 12-match stint.
He will not return to New Zealand rugby after his five-month contract was over and All Blacks coach Steve Hansen said it was tough losing a player who was starting to fulfil his promise in the 15-a-side game.
“It is a shame and disappointing that New Zealand rugby is losing him,” Hansen said.
New Zealand Rugby Union boss Steve Tew said Williams had exercised a right under his contract to leave New Zealand rugby after the Super Rugby competition. He had not ruled out a return to New Zealand rugby in the future.
“We want to wish Sonny all the very best and thank him for the contribution he has made to New Zealand rugby,” Tew said.
Japan’s Top League season begins on August 31, running until the end of January, and it has been estimated he will earn as much as $1.7 million for his time in Japan.
The timing of the Japanese season will allow Williams to make a return to rugby league in 2013.
Williams split in acrimonious terms from the Bulldogs in 2008, just 18 months into a five-year deal to join French rugby club Toulon.
He was part of last year’s Rugby World Cup-winning All Blacks team, and showed commanding form in the three-Test series against Ireland last month.
Williams is also expected to continue his professional boxing career.