The Wallabies tamed Sonny Bill Williams and thrashed a disjointed Barbarians side to open their international season with a 55-7 victory at the Sydney Football Stadium on Saturday night.

Showing some early season rustiness against an even less cohesive Baa Baas, the Australians scored eight tries, with winger Drew Mitchell grabbing a double, to post their biggest victory in 11 games against the invitational side in front of 39,688 fans.

The Wallabies led 20-7 at the break after three first-half tries, before running away with the match, the famous Barbarians' first on Australian soil, in the second 40 minutes.

Man-of-the-match Matt Giteau booted four conversions and a penalty goal on top of a pinpoint kicking game in general play.

Playing his first game in Australian since walking out on NRL club the Bulldogs, Williams looked dangerous on occasions with his trademark offload and only an amazing tackle from halfback Luke Burgess prevented him scoring with his first touch in international rugby in just the third minute.

But the former NRL superstar was also on the end of plenty of attention from the Australians, most notably from opposite number Stirling Mortlock, who smashed him in a big tackle two minutes later.

Williams also had a hand in the Baa Baas' first-half try, scored by Iain Balshaw on the stroke of halftime.

He was also booed by a small section of the crowd wearing blue and white when he cleaned up by a Lachie Turner grubber in the second half.

The Wallabies have injury concerns over Burgess (hip) and Matt Hodgson, who left the field in the first half of his first game for Australia with an AC joint problem.

Robbed of Australian star Rocky Elsom, who withdrew with a leg injury on match day, the thrown-together Barbarians attempted to play with their trademark flair but predictably struggled for combination and pace.

The Wallabies opened their scoring through James Horwill in the sixth minute, before Mitchell got his first in the 16th and Giteau dummied twice and strolled over untouched in the 23rd.

Their attack lost some momentum until the 57th minute, when hooker Stephen Moore finished some good work from George Smith and Josh Valentine to push the floodgates open.

Prop Ben Alexander strode over in the 66th minute, Mitchell got his second in the 72nd, while David Pocock and James O'Connor bagged tries in the final three minutes.

Australia take home the new Sir Nicholas Shehadie Cup with the win.

Mortlock said Williams "held his own" while Wallabies coach Robbie Deans said the step up had been huge for the Kiwi.

"The toughest thing for Sonny was going essentially from club rugby into an international," Deans said.

"You just don't get the license to play and you could see he was often looking to offload and play a game that you can probably get away with at club level, but you just don't here because the channels close and the contact is more significant."

Deans was pleased with the first hit-out but will look for more patience from his side.

"It was a good start, there was some really good stuff," he said.

"We were a little bit impatient, put the ball on the ground a little bit too frequently where if we'd just been a little bit more patient we would have profited."

Barbarians captain Phil Waugh, who will join the Wallabies squad on Sunday morning, was impressed with the Australians.

"They certainly finished off well ... that pressure into points really hurt us," he said.

Deans confirmed Elsom would also join the Australian squad on Sunday and his injury would be assessed by medical staff.

Hodgson is likely to spend a lengthy time on the sidelines, while Burgess' injury is considered minor.

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