Former All Blacks coach Graham Henry is confident Sonny Bill Williams is only leaving rugby for a short time and believes the dual international will retire alongside Brad Thorn as an all-time great in both union and league.

Williams was part of Henry's World Cup-winning New Zealand outfit last year, but as he prepares for Saturday night's Bledisloe Cup opener against Australia in Sydney, the coaching guru says Williams has now hit his stride in the 15-man game.

After a spell in Japanese rugby, Williams intends to return to the NRL in 2013 on a rumoured one-year deal with the Sydney  Roosters.

After that, Henry expects the 27-year-old will be on his way back to rugby in New Zealand.

The code-hopping ways of Williams have attracted criticism in some quarters, but Henry vouched for the intentions of one of the most unique talents he's ever coached.

''He's leaving rugby for a short period of time I would imagine to fulfil an obligation ... he's got personal standards, so you've just got to appreciate what he's doing there,'' said Henry, who has  recently released his detailed biography - Graham Henry Final Word.

''He's a very fine player now and he's becoming world class.

''He's had more time in the game, he's got comfortable with the  game and what the game is about.''

Brad Thorn is arguably the greatest dual international of them all.

In league he won premierships with Brisbane, State of Origins with Queensland and played for Australia.

In rugby he's a Super Rugby champion for the Crusaders, Bledisloe Cup winner for New Zealand and finished his international  career a World Cup winner.

Henry believes Williams can enter that same class.

''I think he has that potential,'' said Henry.''I think rugby is a more difficult game to play than league - it's more multi-dimensional ... the league transition will be easy for him, much easier than the rugby transition. I think he'll be a  very effective player in both codes.

''There's not too many backs that have got his physique ... and have his sort of athleticism and skill.

''He's brought a different dimension to the game.

''Henry was highly critical of fallen Wallabies five-eighth Quade Cooper in his biography.

Cooper had a disastrous World Cup last year and Henry believes the barrage of criticism he copped from the New Zealand public was in many ways self-inflicted, following a couple of ''cheap shots'' on All Blacks skipper Richie McCaw.

Berrick Barnes has since taken his No 10 jersey but Henry predicts it will only be a matter of time until Cooper forces his  way back into the Australian side.

''He's a talent, he's a very talented rugby player,'' said Henry.

''OK, he had his challenges in the Rugby World Cup ... but I've got a lot of respect for his ability to play the game and it'll  just be a matter of time until he's back in that team and being a major influence.''

- AAP