Wales' win over Fiji on Thursday means co-hosts England - beaten by Warren Gatland's men in their previous Pool A match - must win to say alive in the tournament.
England boast a strong recent record against Australia, having won four of their last five meetings with the Wallabies, including home victories in November 2013 and 2014.
And after the Wallabies' scrum was monstered in their most recent trip to Twickenham, Cheika concedes his pack has a point to prove.
"I know they (England) think we're weak in the forwards," he said.
"It's pretty obvious that they're saying it out loud. They've done it to us; they've stuck it to us the last couple of times so there's nothing we can say in our room that's going to make any difference.
"The only place things are going to be different is on the field on Saturday night and that's where we've got to show our colours. Talk's cheap, you know."
There has certainly been plenty of chatter ahead of Saturday's game, with former Australia coach Eddie Jones and England back Danny Cipriani among those to have made bold predictions.
Cipriani cheekily suggested not one Australian would be worthy of a place in England's team, but fly-half Owen Farrell is not underestimating the threat posed by Cheika's side.
"You only have to look at the Rugby Championship (which Australia won) to see how good a team they are," said Farrell.
"They've got some real threats within that team. Within the backline they've got some players that can hurt you if you switch off.
"I guess our task is to stay concentrated for 80 minutes, stay in the game and make sure we don't dip in and out of it, because if that's the case we will get hurt. We have full confidence in ourselves that we can do that and a massive belief that we can perform this weekend.''
Australia have reverted to the same XV that beat Fiji 28-13, after making wholesale changes for their clash with minnows Uruguay.
Lancaster has restored the fit-again Jonathan Joseph to his England side, at the expense of Sam Burgess, while Ben Morgan and Joe Launchbury replace injury victims Billy Vunipola and Courtney Lawes.