Wales have lost 10 matches in a row to the Wallabies, but each of the last nine have been by nine points or less.
They meet again at Twickenham on Saturday to determine who tops Pool A.
"The boys have spoken about how much heartache we've had against Australia so we have to go in with huge motivation," said the fly-half, 30.
"Everyone is beatable, it's just about getting it right on the day."
Australia won a thrilling match 33-28 in Cardiff the last time the two sides met, on 8 November 2014.