The Wallabies miraculously held on for a 15-6 win to secure top spot in Pool A and a smoother path through the World Cup play-offs as they defended gallantly for almost an entire half of rugby.

At the same time they extended their winning streak against Wales to 11 games in a row and ignited their World Cup hopes without a try being scored.

Wales got over the line three times in the second half but on each occasion there was a gold jersey holding the ball up and it was 111-Test veteran Ashley-Cooper who changed the game.

With the 13-man Wallabies under the pump and leading by six, Wales appeared certain to score in the 70th minute when Ashley-Cooper rushed up in defence to smash playmaker Dan Biggar.

Kurtley Beale then rushed in and won an unlikely penalty which eased the pressure and allowed the Wallabies to hold on for a historic win.

"Given the circumstances, two men down in a big match in the World Cup, our backs against the wall, it was one of the gutsiest wins I've been involved with," Ashley-Cooper said.

"No one gave up. Everyone here involved is really proud, even guys in suits who weren't [playing], the expression on their faces and the high-fiving. It was really something to be part of.

"We just gave it everything and there was great character shown … We're not getting ahead of ourselves. But if we are to go all the way, then that period in defence could be seen as a pivotal moment."

The Wallabies' defensive effort has been described as "extraordinary", "courageous" and "gutsy" as they constantly repelled attacking waves.

They were starved of possession and made twice as many tackles as Wales. But still they refused to wilt under pressure.

All World Cup teams have a defining moment in their campaign. For Australia's 1999 campaign it was Stephen Larkham's field goal. For the 2007 tournament it was Australia's backwards scrum. And in 2011 it was Quade Cooper's New Zealand struggles.

But if the Wallabies go on to make it to the World Cup final, it will be their defensive effort against Wales that will provide the foundation.

They will play Scotland in a quarter-final at Twickenham on Sunday (Monday morning AEDT) before taking on either Argentina, Ireland or France in the semi-final.

They have locked in their place on the opposite side of the draw to powerhouses New Zealand and South Africa and have a perceived smoother run through the play-offs.

But Cheika is refusing to allow the players to start dreaming big.

"We're just keeping our feet on the ground and not getting ahead of ourselves. It's a long tournament, we've got seven hard Tests in a row and we just can't afford to slip up in any of those games or your World Cup is over," Ashley-Cooper said.

"We've done extremely well to get where we are now. Hopefully we've still got three weeks left."

There were concerns the Wallabies would be underdone after a warm-up game against the United States and then their first two World Cup matches against Fiji and Uruguay.

But they have shown over the past two weeks they are battle hardened, and back-rower Scott Fardy said they would carry that into the finals stages.

"It feels like that for us anyway. Nothing changes, we knew we had to come here and win seven games to win it," Fardy said.

"That's been the goal from the start, you come over here win seven and then go home. That's the plan. We're not looking further ahead than Scotland though."