The Wallabies and Wales are fighting for top spot in pool A and a smoother run to the World Cup final when they clash at Twickenham this weekend.

The winner will play against either Scotland or Japan in the quarter-finals, while the loser faces likely back-to-back matches against South Africa and New Zealand.

The Wallabies are riding high after a 33-13 win against England last weekend and are heavy favourites to beat Wales for the 11th straight time.

But after the Wallabies' yoyo form against New Zealand in August, Cheika is taking no chances and declared momentum would not be a factor against Wales.

The Wallabies beat the All Blacks in the opening Bledisloe Cup match earlier this year to breathe new life into Australian rugby.

But they were embarrassed a week later when Cheika made several changes before a 41-13 Eden Park mauling.

Asked what momentum the Wallabies could take from their rousing win against England, Cheika said: "None. We supposedly had momentum after we beat New Zealand in Sydney and then they put their boot to our backside.

"That's what can happen if you are not 'on' every single day. Today is a day we have got to be better and enjoy that.

"Make it tough for ourselves but enjoy being in that situation and learn from those times. It is a game we are really looking forward to because it is another step along the way.

"I think the team wanting to move forward and grow as a team needs to be able to take these challenges on and step up and improve. That's something to work on for us because we have not been in that space before."

Cheika has called Test rookie Sean McMahon into the starting team to replace Michael Hooper, who was suspended for one week after an illegal ruck cleanout on England's Mike Brown.

Israel Folau has passed a fitness test and will start at fullback, while Drew Mitchell replaces Rob Horne on the wing. Dean Mumm has been promoted to the starting side ahead of Rob Simmons.

Cheika rubbished suggestions finishing top of their pool would give the Wallabies an easier path to the final.

But being able to avoid showdowns with South Africa and New Zealand would be a major boost.

"This is going to sound maybe a bit silly but [it's] not necessarily win the group, even though one's a consequence of the other," Cheika said.

"I just want to try to win every game, like individually. No team has ever won the World Cup after losing a game and if we want to work on our improvements each week, which you know in tournament play you have to do.

"I don't subscribe to theory 'you win the pool you're going to get an easier run', because that's disrespecting the opposition and that's not what we're about."

The Wallabies haven't lost to Wales since 2008 and can heap more misery on the Red Dragons on the weekend.

Cheika said the challenge for the Wallabies was to "be great" and learn from their disaster against New Zealand two months ago.

"You've got to go try to be great and to do that you've got to keep backing up the good bits every day," he said.

"And so we weren't on that occasion [in the loss to New Zealand] and the one thing you've got to take from it is to learn the lesson about improving all the time, even if things go your way. That's why we're harping on about it so much because we really believe it."

Wallabies team: 15. Israel Folau, 14. Adam Ashley-Cooper, 13. Tevita Kuridrani, 12. Matt Giteau, 11. Drew Mitchell, 10. Bernard Foley, 9. Will Genia, 8. David Pocock, 7. Sean McMahon, 6. Scott Fardy, 5. Dean Mumm, 4. Kane Douglas, 3. Sekope Kepu, 2. Stephen Moore, 1. Scott Sio. Reserves: 16. Tatafu Polota-Nau, 17. James Slipper, 18. Greg Holmes, 19. Rob Simmons, 20. Ben McCalman, 21. Nick Phipps, 22. Matt Toomua, 23. Kurtley Beale.