After both sides beat England over the past few weekends, Wales and Australia head to Twickenham with the pressure off knowing each of them will take their place in the knockout stages.
Whoever loses the game will come second in the group and face South Africa in the quarter-finals, with Scotland or Japan awaiting the winners showing the importance of this game.
Warren Gatland and Michael Cheika each masterminded wins over the Red Rose in recent weeks, and it will be a fascinating meeting of minds with a lot at stake.
North moves to centre amid Welsh re-shuffle
George North has come in off the wing, and will start at outside-centre at Twickenham, where he will play outside Jamie Roberts.
Injuries to Jonathan Davies, Cory Allen and Scott Williams have depleted the Welsh centre options, leading to North's move inside, with Liam Williams taking his place on the wing, and Gareth Anscombe slotting in at full-back.
Justin Tipuric is given a chance to start in place of Dan Lydiate, with Paul James starting his first game of the tournament, which should help improve the Welsh scrum which has been a complete mess so far.
McMahon replaces banned Hooper as one of three changes
Sean McMahon has been chosen ahead of Ben McCalman as the man to take Michael Hooper's spot in the back-row after the Waratahs flanker was banned for one week after his dangerous clear-out of Mike Brown.
Exciting youngster McMahon was awesome in the win over Uruguay at Villa Park, and coming off the back of a brilliant season in Super Rugby where he was a standout player for a struggling Melbourne Rebels side.
Dean Mumm also comes into the starting lineup, in place of Rob Simmons who drops to the bench, whilst veteran Drew Mitchell takes the injured Rob Horne's spot on the left wing.
Israel Folau has proved his fitness and will start at full-back, with Matt Giteau also overcoming a knock to take his place at inside-centre, where he again joins up with Tevita Kuridrani.
How the sides line up
Wales: 1. Paul James, 2. Scott Baldwin, 3. Samson Lee
4. Luke Charteris, 5. Alun Wyn Jones
6. Sam Warburton (captain), 7. Justin Tipuric, 8. Taulupe Faletau
9. Gareth Davies, 10. Dan Biggar
11. Liam Williams, 12. Jamie Roberts, 13. George North, 14. Alex Cuthbert, 15. Gareth Anscombe
Replacements: 16. Ken Owens, 17. Aaron Jarvis, 18. Tom Francis, 19. Jake Ball, 20. Ross Moriarty, 21. Lloyd Williams, 22. Rhys Priestland, 23. James Hook
Australia: 1. Scott Sio, 2. Stephen Moore (captain), 3. Sekope Kepu
4. Kane Douglas, 5. Dean Mumm
6. Scott Fardy, 7. Sean McMahon, 8. David Pocock
9. Will Genia, 10. Bernard Foley
11. Drew Mitchell, 12. Matt Giteau, 13. Tevita Kuridrani, 14. Adam Ashley-Cooper, 15. Israel Folau
Replacements: 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