Wallabies turn to McMahon
The Wallabies have been forced into two changes with Rob Horne (shoulder) and Michael Hooper (suspension) ruled out for the match which will determine the winner of Pool A.
McMahon slots straight into the No.7 jumper, the 21-year-old's second run-on selection for the tournament, with David Pocock remaining at No.8 to make his fourth start at the back of the scrum. Pocock will also share the vice-captaincy with Adam Ashley-Cooper.
66-Test veteran Drew Mitchell comes onto the wing for Horne, after a two-try performance against Uruguay earlier in the tournament made him the third-highest try scorer in World Cup history.
Dean Mumm has also been included in the Wallabies run-on team for the second time at the World Cup, pairing up with Kane Douglas at lock with Rob Simmons moving to the bench.
Fullback Israel Folau has overcome an ankle injury to take his place in the side, continuing his good run with the Wallabies having not missed a Test through injury since his 2013 debut against the British and Irish Lions.
Scott Sio is the only player to start in every Test for the Wallabies so far in the tournament, remaining at loosehead prop for the pivotal match.
Wallabies coach Michael Cheika said the team's focus remains firmly on the task at hand.
"Every match in this tournament has required our team to step up to another level and we'll need to do that again against an undefeated Welsh team.
"Every player in our squad knows the improvements required, and we still have two days to continue to make them, both individually and as a team.
"Saturday night at Twickenham is another opportunity for us to put in a solid performance and continue to build towards the next stage of the tournament."
Australia: 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 (captain), 1 Scott Sio.
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.
Date: Saturday, October 10
Venue: Twickenham, London
Kick-off: 16.45 (15.45 GMT, 02.45 AEST Sunday, October 11)
Referee: Craig Joubert (South Africa)
Assistant referees: Jérôme Garcès (France), Stuart Berry (South Africa)
TMO: Shaun Veldsman (South Africa)
