Wallabies coach Robbie Deans is clinging to hope lock Nathan Sharpe's injury-enforced absence from Saturday night's Tri Nations clash with South Africa will be a blessing in disguise.

The Springboks boast the most lethal line-out in the world, with lock Victor Matfield a master at the set piece.

Either Dean Mumm or Mark Chisholm will come in to partner James Horwill in the second row and Deans said the new combination could actually trip up Matfield at the line-out.

"He (Matfield) does his homework, he analyses his opponents well so that's one advantage we'll have this week, he'll have no background potentially," Deans said.

But Deans admitted Australia's new line-out combination would come under intense pressure from Matfield and Bakkies Botha.

"Matfield understands the trade and is a very good reader of body language," Deans said.

"He's a big man and very experienced ... and with that comes composure and belief.

"It brings belief to the whole line-out.

"It brings belief to the bloke throwing it in as well.

"They (Matfield and Bakkies Botha) are a well established combination.

"But they haven't had it their own way and we were able to bring a bit of pressure there last time and we'll look to do the same again."

Australia were dealt a double blow on Tuesday when both Sharpe (shoulder) and inside centre Berrick Barnes (neck/concussion) were ruled out of the Perth Test through injury.

And the Wallabies are still sweating on the fitness of flanker Rocky Elsom, who injured his ankle in last Saturday's 19-18 loss to New Zealand in Sydney.

But Deans was confident utility back James O'Connor (cork) would be fit to play after the 19-year-old ran strongly on Tuesday.

The Wallabies delayed the naming of their squad until Thursday and Deans was giving little away when questioned about the potential make-up of the XV.

However, No.8 Richard Brown did let it slip that Chisholm had been put in charge of the line-out calls during Tuesday's training session.

"He (Sharpe) is our calling lock at the moment so it's a problem when you lose your calling lock but through training today Mark Chisholm filled his shoes and did a good job of it," Brown said.

With Barnes out, Deans could name Ryan Cross at outside centre and shift Adam Ashley-Cooper to fullback, with O'Connor reverting to inside centre to partner flyhalf Matt Giteau in Deans' preferred double-playmaker role.

The Wallabies sit rock bottom of the Tri Nations table with just two bonus points from three games and must win their remaining three hit-outs to have any chance of snaring the title.

"We can take confidence that we aren't far away from having these wins but at the same time we are playing badly and we are still close to the competition," Brown said.

"We are still confident. There's no heads downs."

Deans said he had put plans in place to better counter South Africa's kicking-dominated game plan.

Springboks flyhalf Morne Steyn booted seven penalties and a drop goal to lead his side to a 29-17 win over Australia in Cape Town earlier this month.

"Their kicking game has been remarkably accurate and they've been able to bring a lot of pressure to by isolating players," Deans said.

"It's about forcing them first and foremost to have to play another way and potentially from there forcing them into error."

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