The Wallabies defended with 13 men for 10 minutes and amazingly clung to a narrow lead as they beat Wales 15-6 and finished at the top of Pool A.

Cheika is hopeful David Pocock (calf), Israel Folau (ankle) and Matt Giteau (ribs) will all recover in time for a knock-out quarter-final against Scotland next weekend.

Beating Wales ensured the Wallabies would avoid clashes against South Africa and New Zealand until a potential meeting in the final.

The Wallabies have shown their attacking sparkle and defensive courage in their past two games against England and Wales, proving they're not one-trick ponies under Cheika's guidance.

Cheika was pleased with the way his players survived a "chaotic period" where Wales bombarded the under-manned Australian line and players somehow kept turning up to make tackles.

"I'm happy we won, it was pretty tight. It was a different type of game for us, we had to show a different skin and I'm pleased with the result," Cheika said.

"There are plenty of imperfections in our game and that's something we have to improve before we go into the quarter-final. We definitely don't want to be doing it [defending with 13 men] again. We got away with it this time, but you won't get away with it at this level again.

"You don't get belief from things like, you have to have belief to go through it. The team still needs to build its self-belief through the tournament and bring that back from where it's been. Understanding what that brings and be a bit more resilient.

"Our finishers really did a fantastic job, coming in and doing the work of one-and-a-half men and they all came on in that chaotic period."

The Wallabies are undefeated through four games and are perfectly positioned to make a run to their third World Cup title.

But there were plenty of hairy moments in a try-less clash against Wales, which was brutal in every aspect of the game.

Bernard Foley booted five penalties for Australia, but it was the courageous defence in the second half that will be the foundation for a World Cup bid.

Folau, Kane Douglas, Foley and Ben McCalman made try-saving tackles when Wales was pushing for victory. Will Genia and Dean Mumm were sent to the sin bin, but the Wallabies managed to escape when Adam Ashley-Cooper and Kurtley Beale forced a breakdown turnover.

Wales dominated possession and territory, but still couldn't break a gold brick wall, even when Australia had two players on the sideline.

"We've got a lot of 'work-ons' before we play Scotland because I'm sure they'll attack us in certain areas they've identified," Cheika said.

"We have to try not to get into that situation we were in tonight, that rear-guard. But still be capable of doing it if we need to. [Defence] isn't a secret weapon. They say behind every man is a good woman. Behind every attack you have to have a good defence."

Folau limped through 80 minutes with an ankle injury and had to stay on for the last 10 minutes because there were no reserves left on the Wallabies bench.

Pocock came from the field in the second half with a calf problem, while Giteau put his body on the line with some desperate defence.

"David's got a bit of a bang on his calf. Doesn't seem too bad but at the end I brought him off because I wasn't sure who to bring on or bring off when all that was happening so I had to make a decision to bring someone on," Cheika said.

"Israel, just ankle, he got a bit weary after last week. We did want to bring him off - in a perfect world we would have brought him off with 10 or 20 minutes to go because he hadn't trained all week but we couldn't because of a few things that happened around us. He got through the game but the first signs are pretty much the same as last week."