Williams will then join an NRL club in Australia in 2013 with the Sydney Roosters and Canterbury Bulldogs both linked to moves.
"The decision to come to the Chiefs was a pretty big thing, but it has really helped me, especially off the field," said Williams.
"I didn't think I would love rugby as much as I have.
"If things work out I would love to come back and play for the All Blacks in the future. But I know there is a group of players who are coming through.
"I've chosen my path now I've got to walk it."
While disappointed with the loss Williams, All Blacks coach Steve Hanson has refused to rule out a return to the international set-up in the future.
"He came to New Zealand rugby two years ago as a convert player and was similar to Brad Thorn in that regard," added Hansen.
"Sonny had high level skills and freakish abilities without a fully developed game.
"This season we have seen the birth of Sonny as a rugby player with his game understanding complementing his array of skills.
"His performance in the Test series against Ireland showed that he was world-class."