Kepu has banned himself from extra meals, sweets and given himself an earlier bed time as his way of showing his teammates he's committed to helping Australia chase World Cup success.

And defence coach Nathan Grey is confident the team's courage under duress against Wales last weekend will be the stepping stone they need to achieve their goal.

The under-manned Wallabies scrapped and fought their way to a 15-6 triumph against Wales, booking a spot against Scotland in the quarter finals and avoiding the same side of the play-off draw as New Zealand and South Africa.

Their defence was superb, continually repelling Welsh attacking raids even with just 13 men on the field when Will Genia and Dean Mumm were sent to the sin bin.

Kepu said the Wallabies had faith in their fitness to make sure they didn't fall away in games, and said he's cut out second-helpings and curbed his sweet tooth to be in the best shape of his career.

"We talk about every day, the little sacrifices and discipline," Kepu said.

"For me, when I am craving that extra meal or those sweets … when I'm trying to get myself to bed early, I think of who I am doing it for and who I am going to let down if I do the things I shouldn't be.

"That's what keeps me on track and on path. I know everyone else is doing that in the team, and if you have 31 guys singing from the same song sheet, that's pretty powerful.

"If you're trying to gain fitness now, it's too late. We have done [the work] and we will withdraw from the bank whenever we need to."

Israel Folau, Ben McCalman, Bernard Foley and Kane Douglas all made try-saving tackles against Wales, refusing to give up as they faced an attacking onslaught.

It showed the Wallabies have the attacking prowess and the defence required to win World Cups.

But Grey, who was part of the 1999 World Cup-winning squad, said this Wallabies team is unique.

"It's a little bit unfair to make comparisons between different eras," Grey said.

"We spoke about [tackling technique] and you have to make sure it's spot on, and that's a foundation for you to fall back on.

"But you've got to have that underlying determination and trust in each other."

Asked where the defensive effort against Wales ranked in terms of courage, Grey said: "For us, it's a real stepping stone. The pool stage is over now, it's important we take stock … but we'll lock that up, put it away and get on to Scotland.

"…Being able to absorb pressure is something that you need to be able to do. But let's not make a habit of it.

"Being able to get off the ground and get in the defensive line, it doesn't take a lot of talent. You just have to want to do that. We saw that in spades."

The Wallabies will shift their base from central London to closer to Twickenham on Monday (Tuesday morning AEDT) in preparation for the finals stages.

If they beat Scotland, they will face either Ireland or Argentina in the semi-final.

The Wallabies are battered and bruised after withstanding the Wales onslaught. David Pocock (calf), Israel Folau (ankle) and Matt Giteau (ribs) all pushed through the pain barrier for the pool match.

But Kepu says the players will draw on intense training sessions at a two-week camp in the United States to know they can handle whatever is thrown at them.

"There were days when we were in America that we still laugh about: 38 degrees and we had to throw on [tackling] suits. It felt like 50 degrees in them and we were just smashing each other for 30-40 minutes and solving all those problems," Kepu said.

"All the training has been about getting us fatigued and testing us under those moments when we are really buggered.

"When I was going through a bit of pain [against Wales], you know you're hurting but you know the opposition is hurting as well, if not worse.

"I just have full confidence in the work we have done but we have to keep building and not let it slip."

WORLD CUP QUARTER FINALS

Quarter final one: South Africa v Wales at Twickenham, 2am Sunday AEDT.

Quarter final two: New Zealand v France at Cardiff, 6am Sunday AEDT.

Quarter final three: Ireland v Argentina at Cardiff, 11pm Sunday AEDT.

Quarter final four: Australia v Scotland at Twickenham, 2am Monday AEDT.