England captain Charlotte Edwards says her team must not underestimate Australia, despite being on the verge of retaining the Women's Ashes.
Winning Thursday's Melbourne one-dayer would give them an unassailable lead, with four matches still to play.
"The team are very confident but we know we're playing against a very good Australian side," Edwards said.
"We have to take it game by game and not be complacent but hopefully tomorrow we can win the Ashes."
England have already won the single Test match between the sides in Perth, and the first of three one-day internationals. The series concludes with three Twenty20 matches.
With six points for the Test win and two for each limited-overs match, the tourists can take their tally to 10 points with victory at the Melbourne Cricket Ground.
Edwards confirmed that fast bowler Katherine Brunt will miss the match because of a back injury.
Australia skipper and wicketkeeper Jodie Fields is still a major doubt with a fractured finger.
If she does not pass a late fitness test, batter Meg Lanning, 21, will deputise, having become the youngest captain of a men's or women's Australia cricket team when deputising for Fields in the first ODI.
England wicketkeeper Sarah Taylor told BBC Sport: "It would be amazing to do it in Australia and here [at the MCG]. That's big for us and what's creeping into the back of our mind. We just want to go and do it.
"There is everything still to play for. [Australia] can still win from here and we will be going hard."