Wallabies head coach Michael Cheika expects a brutal onslaught from Wales when the two teams meet in their Pool A showdown at Twickenham on Saturday.
Both sides have qualified for the quarter-finals and know that victory will secure them top spot in the pool while the losers will have to face a resurgent Springbok side in the play-offs.
"I feel it will be the most brutal game we have faced so far," said Cheika.
"It has been physical so far. My knowledge of tournament play is that the longer it goes on, the more brutal it gets."
Wales know a victory would be a massive boost for northern hemisphere rugby as the quarter-final clash between Australia and South Africa will mean that one of the southern hemisphere's big teams will be knocked out.
Cheika has been forced to make a change to his back row with 21-year-old Sean McMahon replacing Michael Hooper at openside flanker. Hooper is serving a one-match suspension for using his shoulder illegally at a ruck in Australia's 33-13 victory over England last Saturday.
Cheika is confident McMahon will give a good account of himself in his battle with Wales skipper Sam Warburton.
"I'm sure he will enjoy himself," added Cheika.
And I am sure he will bring a lot of energy to the game.
"Sean has played very well in the tests I have seen him play. He works hard and it's a good opportunity for me to let him shine on this stage."