Wales have rejigged their line-up to have George North partner Jamie Roberts in the centres in one of the biggest partnerships in the World Cup.

They weigh a combined 217 kilograms and will target 84kg Giteau in the back line to try to make inroads in the Wallabies' defence.

But Giteau says Australia is prepared for the attacking onslaught.

"They use Roberts as their go forward, traditionally that's just what they've done," Giteau said.

"But they've brought North into the centres as well to get a little bit more ball. He probably hasn't seen as much as he would've liked in their first few games. He's obviously a dangerous runner.

"They did the same against Australia last year, I was commentating that game. They obviously see it as an effective weapon against Australia."

Giteau will play his 99th Test in the crucial pool match, with the winner earning a smoother path to the World Cup final.

The Roberts (110kg) and North (107kg) combination will certainly test Giteau's defence and throw their 20-plus kilogram advantage around after the playmaker injured his ribs last week.

But the Wallabies are confident they have the defensive structures to combat Wales' attacking weapons.

"You can't hide in Test football … [Rob Horne] got injured and you've just got to fill that void," Giteau said.

"We train for those scenarios … you've just got to find a way to deal with it."

Wallabies five-eighth Bernard Foley blitzed England with a record-breaking 28-point haul last weekend.

He also scored 18 points against Wales last year and slotted a drop goal that sealed the match for the Wallabies.

But the Wallabies are also wary of Wales linchpin Dan Biggar, who boasts a near-flawless kicking record and has stepped up to be the team's leader.

Wales coach Warren Gatland says his side is "desperate" going into the pool A decider.

The winner will book a quarter-final berth against either Scotland or Japan and also avoid South Africa and New Zealand until the final.

Wallabies coach Michael Cheika did not want to be drawn on the easier path, saying it was disrespectful to the opposition.

But Gatland conceded beating the Wallabies would blow the draw wide open for Wales.

It took Gatland 23 attempts to beat one of the southern hemisphere's big three – Australia, South Africa or New Zealand.

"We are desperate to win this game, as you win this group and potentially your road through to quarters, semis and final is a little bit easier than the other side of the draw," Gatland said.

"We are desperate to finish off well and win this group, and hopefully find ourselves potentially a second seed team in the quarter-finals.

"The last five times against Australia there has been a score in it. We've been ahead but not been able to finish it off. You learn from that."