Dual international Michael O'Connor has questioned whether Sonny Bill Williams has the speed to be a top class rugby union centre.

O'Connor, who played 12 Tests for the Wallabies and 17 for the Kangaroos, was generally impressed with Williams' performance for a Barbarians team outplayed to the tune of 55-7 by the Wallabies on Saturday night.

But the former centre said genuine pace could be Williams' Achilles heel at No.13.

"I think centre at the moment is good (for Williams), certainly he's a strong defensive player," O'Connor told AAP.

"The only thing you'd say is speed may be an issue."

O'Connor said Williams would learn to overcome defensive lapses such as the one which let Matt Giteau slice through for a try untouched at the Sydney Football Stadium.

"He hasn't put himself in that situation as many times as Stirling Mortlock and, when we scored that try, Giteau goes through and he's probably drifting off onto the wrong man," O'Connor said.

"He'll learn from that experience and really all he needs is a lot more rugby.

"He certainly is a footy player.

"He showed, certainly with that first break, that given a bit of ball he's dangerous, he's got a good offload.

"Given the amount of rugby that he's played I thought he did OK.

"I'd just be playing more rugby, that's all it is, it's just putting yourself in situations to be able to anticipate play."

Any move to the forwards for Williams, though, could be made more difficult by the game's decision not to adopt the free kick sanction for most breakdown infringements.

"That tap and go where you've got roll-on play which would have suited him is gone and the technical aspects of forward play are really back on the agenda," former Wallabies coach John Connolly told AAP.

"The rolling mauls and so forth ... I wouldn't think that would suit him."

Connolly said the expectations were too high for 23-year-old Williams' international debut.

"He's played a dozen games of rugby for a bottom club in France that was poorly coached and it's a huge step up," Connolly said.

"He was always going to be a PR dream but the reality of it all is he was always going to struggle playing."

Meanwhile, the Wallabies have backed the Baa Baas' first visit to Australia as an ideal way to start their season.

"As far as should we have it each year, I think so ... I think it's a great product for rugby," five-eighth Matt Giteau said after the match.

The Australian Rugby Union was "delighted" with the interest in the match, an ARU spokesman said.

"While we are yet to discuss future matches, there is no doubt we would be open to continuing Barbarians tours to Australia," he said.

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