The Wallabies suffered a shock 9-8 defeat to Andy Robinson's Scotland at Murrayfield, but Roberts, who fractured his skull in a clash with Stirling Mortlock in last season's 21-18 triumph over Australia, admits he expects another battle royale next Saturday.
"It was a huge win for Scottish rugby and the weather conditions suited them down to the ground," said Roberts, who was named HSBC Player of the Tour when the Lions travelled to South Africa this summer.
"But that result will make it tougher for us. It's always harder to play a side who have just lost.
"Australia will come here like a wounded animal and play with extra fire. It's a challenge we are looking forward to."
After a Shane Williams-inspired 33-16 win over Argentina, Wales head coach Warren Gatland has also admitted his side need to raise their game for the showdown with Robbie Deans' men at the Millennium Stadium.
"We will definitely have to step up a level next week," said Gatland, who joined Roberts and co as forwards coach with Ian McGeechan's Lions.
"Last year we beat Australia and we deserved the victory. But, having dealt with Australians before, I know they will be smarting from that defeat. It will make them even more determined to win.
"It will be nice to show another level of work in training and in our performance next weekend to make sure we improve from game to game."