Sonny Bill Williams won't find a better stage to show he's future dual international material than he gets in Sydney next week, says former All Blacks halfback Justin Marshall.

Marshall believes Williams' appearance in a star-studded Barbarians team against the Wallabies at Sydney Football Stadium on June 6 will give a good indication whether he has the potential to become an All Black.

Playing in France, Marshall has been able to monitor the progress of the former New Zealand rugby league Test ace since he controversially switched codes last year to join French club Toulon.

He already rates Williams a successful convert but says the powerhouse 23-year-old gets a platform to show how far he could go when he plays in a world-class Barbarians outfit boasting some 850 Test caps.

"There's huge amount of experience in the team, so what he doesn't now have to worry about is players around him. So he basically is able to go out and play," said Marshall.

"I always found it easier playing for the Crusaders (in Super 14) than for (province) Canterbury. I found it easier playing for the All Blacks than what it was for Canterbury because the quality of players around me was that much better.

"And I think in an environment like that, he will find that it's easier for him to show the skills that he has.

"So it certainly will be an indication of where he can go.

"I don't think it's necessarily an indication of where he's at, but it will show his potential more than anything.

"It's obviously a stepping stone to above the (level) he's progressed to now."

Marshall says Williams' transition has been the more impressive given it has required a move from the back row, where he played most of his league, to the centres in rugby.

"He's adjusted really well to the game," Marshall said.

"I was involved with Montpellier when he was playing for Toulon in the Top 14, so I saw a lot of him playing and experienced playing against him.

"It's difficult, I think, to make the transition that he has from forward to back.

"Guys like Gasnier and Gower, who have (also) made the move more recently, were (always) backs. Yes, the positional requirements are different and tactically it's different to rugby league.

"But when you move someone from playing in the forwards to the backline in rugby and compound that with the rules, I think he's adjusted very well.

"He's an athlete, he's got brilliant ball skills, he's got uncompromising physical presence about him and I honestly think he'll only get better.

"He's going to be a bit of an acquisition for us next week. I'm glad he's on my side, rather than the opposition's, that's for sure.

"I'm really looking forward to playing with him."

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