Australia's tallest and smallest must rise highest at Subiaco Oval for the Wallabies to end their worst losing streak in four years and keep the Tri-Nations alive.

The Wallabies lineout and diminutive teenage fullback James O'Connor have the biggest assignments against a confident unbeaten Springbok outfit on Saturday night.

Those challenges have been made even more difficult by Perth's inclement weather, which will suit the world champions' highly-effective high kick-and-chase tactics.

More showers are forecast for the match, where 19-year-old O'Connor must leap high with surety to survive a bomb barrage on his Western Force home ground.

The Wallabies ($2.55 with bookmakers) have never been such big underdogs at home against the Springboks and a fourth straight loss will see them surrender the Mandela Plate as well as virtually hand South Africa the Tri-Nations crown.

Wallabies Mr Fix-It Adam Ashley-Cooper admitted the wet will aid the visitors but confidently backed his fullback successor and wingers Peter Hynes and Lachie Turner to deal with the big chasers who continually turned the ball over in the 29-17 Cape Town loss.

"Like they approach any Test, there will be a lot of kicking and I think the game will be won tomorrow night with our back three dominating that high ball and securing the pill," said the new inside centre.

"(O'Connor) is a great little talent and he's looking forward to the challenge, he's got two great wingers alongside him that are going to help fight that battle.

"He's in front of his home crowd and he's very comfortable with it."

South Africa are on guard for O'Connor's X-factor after he was brilliant off the bench at Newlands.

"Under the high ball he's been playing very well so we will have to be spot-on to put him under pressure otherwise he can hurt us," said vice-captain Victor Matfield.

Matfield will again be the man who presents the biggest danger to the Australian forward pack after dismantling the Wallabies lineout three weeks ago.

Mark Chisholm has been rushed straight into the starting side for the injured Nathan Sharpe and will also call the set piece alongside fellow 2m-tall lock James Horwill.

Chisholm has indicated Australia will be much quicker in organising and executing their throws to cut down Matfield's time to counter.

Enforcer Rocky Elsom is back at the front of the lineout in a major boost and he preached composure and confidence to ensure against another meltdown.

Matfield rated the Newlands display one of South Africa's best-ever lineout performances and is ready for different tactics in Perth, where he made his second Test start against John Eales in a 14-all draw in 2001.

"I think we did really well against them at Newlands, probably the best we've ever done against an Australian lineout," he said.

Elsom expects to make more impact than in his impressive return from injury despite nursing his problem ankle this week.

No one need tell him Australia hasn't endured such a poor run since he was among Eddie Jones' 2005 squad which lost eight of nine Tests.

"There's always a lot of pressure, particularly in these matches because you are playing the best sides in the world over and over, so if you don't perform you might lose but also it could get embarrassing too," Elsom said.

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