New Zealand rugby legend Jerry Collins has backed Sonny Bill Williams to become an All Black, but also demanded the critics lay off the former NRL star as he learns the game.

Williams completed his first training run with the Barbarians on Tuesday as he prepares to make his debut on Australian soil in the 15-man game against the Wallabies on Saturday night.

Williams ran at outside centre with the invitational side, which also includes Australians Rocky Elsom, Phil Waugh and Chris Whitaker, at North Sydney Oval.

Asked if he could be a future All Black, the laconic Collins, who played with Williams at French club Toulon this year, said: "Yeah, pretty much."

And the 48-Test veteran backed Williams for his international rugby debut at the Sydney Football Stadium.

"He'll go good mate, no worries, he'll be fine," Collins said.

"He's up to it, he wouldn't get invited to come and play if the club didn't think he was up to it.

"I'm not a critic, I played with the guy, he should be fine."

But Collins warned against placing unrealistic expectations on the former Kiwi second-rower.

"He's only been playing for seven months, hopefully he'll be playing for longer than that so you guys have just got to lay off him and not give him too much of a hard time," the blond-topped No.8 said.

"A lot of guys play rugby for years and don't play their first Test matches until they've played for three or four years.

"You're asking a guy who's played for seven months to come in and play like a veteran.

"He'll go good if we play well. The better we play as a team the better the individuals will go."

Collins said Williams' development at Toulon had been impressive.

"He's a great specimen of an athlete so anyone who's an athlete can pick up things pretty fast," he said.

"He's got those kinds of gifts there that you can't teach ... but in terms of rugby union he's still learning the game and he's just going to get better with time."

The Barbarians will name their team on Wednesday.

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