The worst kept secret in sport has been confirmed - Sonny Bill Williams has ditched the All Blacks to take up a lucractive contract in Japan.

Williams, who burst onto the New Zealand domestic scene in 2010 before winning a World Cup winners medal with the All Blacks last year, has also been heavily linked with a return to the NRL since he first signed with the NZRU.

And at a media conference today 26-year-old Williams, flanked by mentor Wayne Smith and All Blacks' assistant coach Ian Foster, denied he has already signed with Roosters but says it is his intention to play in the NRL next year.

However after his short stint with the Panasonic club in Japan Williams, who walked away from the Bulldogs in 2008, is almost certain to play with the Sydney club.

He is expected to earn over $1.7 million for 12 games in Japan and could earn more than $600,000 with the Roosters - but with his boxing career also taking off it has the potential to rise even further.

The move to Japan - which came about because of a handshake agreement some time ago - rules Sonny Bill out of the forthcoming Rugby Championship - and leaves All Blacks coach Steve Hansen with a midfield issue.

Williams was the first choice during the three-Test series against the Irish with Ma'a Nonu struggling for form for the Blues.

He impressed during the series and would almost certainly have been one of the first names on the team-sheet come the test matches against Australia, South Africa and Argentina.

It is not known whether Sonny Bill will follow another code switcher - Brad Thorn - and return to New Zealand rugby once his Roosters contract has finished.

Williams wouldn't rule that out saying he had loved his time in New Zealand, especially with the Chiefs, but acknowledged there were a number of good players coming through.

But there will be one man smiling at the prospect of Sonny Bill's possible NRL return - Kiwis' coach Stephen Kearney is likely to have one of sport's biggest names available to him for a couple of years.