The hosts go into the series opener at Trent Bridge as the overwhelming favourites to win the tour, and are looking to win their third Ashes in a row.
However, Stewart feels that England should consider past Ashes Tests that have seen them enter the series as the popular choice for victory in order to prevent selling the Aussies short.
"I've heard a few pundits predicting a 5-0 whitewash for England but, although I like their confidence, I think they are seriously underestimating this Australia side," Stewart told BBC Sport.
"In Shane Watson, Chris Rogers and captain Michael Clarke, they have three batsmen who have been around for some time and will be tough to get out in any conditions. In Brad Haddin, they have a real Australian battler who will not be giving away an inch and they also have a strong pace bowling attack.
"Anyone getting over-confident should remember 1989. The tourists were tagged as the worst Australia side ever and then they whipped England 4-0. Pay respect to the opposition and make sure as an England cricketer you have your own game in order."
The first Test gets started at 11am from Trent Bridge.