Admitting they were "spooked" by Fiji, veteran prop James Slipper feels the Wallabies have fixed their weaknesses as they look to keep their World Cup alive with victory over Wales.

A loss in Lyon on Sunday (Monday AEST) would see Eddie Jones's men likely bow out of the tournament before the quarter-finals for the first time in history.

The Australians were badly outplayed last round by Fiji, who swapped out their regular expansive style for a crash-and-bash mission up the middle.

Slipper said the unbeaten Welsh would have taken notes on how the Fijians dominated at the breakdown to put the Wallabies under pressure, with the scoreboard ticking over through forced penalties.

"You've got to give Fiji credit, they played a really physical game and I'm sure a lot of people were probably expecting them to play a little bit differently, a bit more expansively, but they went front door," the four-time World Cup prop said.

"We did get spooked and obviously the scoreboard started ticking over, a bit of pressure was coming on, we were losing the breakdown, we were putting ourselves under pressure.

"I was really impressed with how the Fijians played, they took the game away from us."

Jones has added more size at the breakdown, moving Tom Hooper to openside flanker and Rob Leota to start at blindside with Rob Valetini again at No.8.

Slipper said it was a battle they needed to win to get the jump on Wales, who downed Australia in a pool game at the 2019 World Cup.

"It's something that we have got to fix ... we didn't match their physicality at all and that comes off the back of the breakdown," added Slipper, who has 132 Test caps.

"If you're not winning up front, you're not winning the breakdown, you're not going to win the game and I can tell you now the Welsh will be sitting there looking at that as well."

He said they had tried to train under pressure at their Saint-Etienne base because that's what they expected from Wales from the get-go. In 2019, Dan Biggar slotted a drop goal after just 36 seconds.

"Test match rugby is about pressure, it's about the team that can absorb the pressure the best," he said.

"Ideally, we want to start fast, we want to get ahead, but realistically that always doesn't go to plan as Wales are a good team.

"It's about making sure that we go out there and get our mindset right.

"This is my third World Cup where I have lost a pool game but we've managed as a team to make it back through the finals and do reasonably well.

"You just have to name it how it is - it's a must win game for us, we all know that."

Slipper said despite their poor results under Jones, with just one win from seven Tests, he shared the coach's confidence they would down Wales.

"I've got huge amounts of confidence in this group; everyone knows we've got to win to stay here so if motivation was a factor it's pretty high.

"But it's one thing saying it, there's another thing doing it, and that's probably the theme this week."