Stirling Mortlock sees no need to remind Sonny Bill Williams that shoulder charges are illegal in rugby union, but uppercuts are another matter.

Williams was infamously sin-binned on his rugby debut for French club Toulon last year after shortening up second division player Laussucq Arnaud with one of his NRL-trademark shoulder charges in a pre-season "friendly".

Arnaud described the hit as "like an electrocution".

Wallabies captain Mortlock has no concerns about a similar fate when he marks the 23-year-old ex-Bulldogs giant in Saturday night's Sydney Football Stadium clash with the Barbarians.

"I think he learned that pretty quickly in union, that shoulder charges are illegal," Mortlock said with a grin on Friday.

"And (it's) just as lucky that uppercuts are pretty illegal too."

Mortlock was referring to Williams' bout on the undercard of good friend Anthony Mundine last week.

"At the end of the day Sonny's a world class athlete, he's proved his worth in league and he's been going very well overseas in union," Mortlock said.

"Last week he was in the ring doing a bit of damage there as well so I've got no doubt that he'll be looking forward to the opportunity of playing at a bigger stage in rugby and he'll be up for it."

The match-up at outside centre is one of the most anticipated of the international season opener, not least from Bulldogs fans still angry about Williams' walk-out last year.

"I've had a few people come up to me and say put a shot on him for me or whatever but I think it's all a bit tongue in cheek really," Mortlock said.

"It certainly is from my perspective quite different.

"We've had a fair bit of footage from Sonny playing at Toulon.

"I'm not really focused on going at one player. Usually I'm more focused on my role to play in the group, from my perspective making sure I link in well with the guys around me in the backline."

Mortlock said the unpredictability of the Barbarians would be an ideal test for the Wallabies in their first match of 2009.

"You're not too sure of what to expect but that's probably a good thing for us," Mortlock said.

"It's our first match, we've got to have a real focus on being as cohesive as possible in our first outing so it just brings back the focus on ourselves and what we're about.

"You want to be tested across the paddock, you want to have teams trying to counter-attack against you because the best teams in the world do that whenever they see opportunities.

"So, from our perspective, I guess that's why this first-up match is a great one, we've got no doubt we'll be tested."

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