It was all Wales in the opening 10 minutes but this crunching hit from the Melbourne Rebels star shifted the momentum.
VIDEO: Wallabies forced to play rugby league McCalman channels George Gregan
— Sportal (@SportalAU) October 10, 2015
McMahon was selected to replace suspended flanker Michael Hooper.
The winner of the Pool A clash will play Scotland in the quarter finals next weekend, while the loser faces South Africa.