Head coach Simon Raiwalui has promoted Tuisova to the starting line-up at inside centre, with Semi Radradra moving to the left wing and Vinaya Habosi dropping to the bench.
Against Wales, Tuisova scored a late try after replacing Habosi on the wing midway through the second half. Radradra started that match in the No12 jersey.
"We want to give Josua a go. He's a big power-runner," said Dewes.
"We see Australia as a team that has a lot of power-runners as well and we want to bring some of our biggest power-runners online from the start."
A defeat on Sunday at the Geoffroy-Guichard stadium in Saint Etienne would almost certainly end Fiji's hopes of reaching the quarter-finals.
Dewes said "every game is double-or-quits" for Fiji.
He said they will be looking to get the basics right against the Wallabies, something they failed to do against Wales.
ERRORS DESPITE DOMINANCE
The Flying Fijians produced an error-strewn display in a match in which they dominated possession, territory and ball-carrying.
"For us, reflecting on last week's game, if we had done a few things, nailed the basics, we would have nailed those opportunities that we had," Dewes said from the traditional eve-of-match training session in Saint Etienne, where the players warmed-up by playing football.
"So it's the same thing, nothing changes, just do the basics and our natural game will flow.
"Those offloads will come into play when we get good front ball. It's just playing the natural Fijian style."
Fiji have worked to improve their forward pack in recent years, something the Australians alluded to earlier this week.
"Over the past few years we've had a real emphasis on building our set-piece as a forward pack and trying to build our game around that," said lock Isoa Nasilasila.
"In a rugby game, you can't really play without the set piece, so, individually, we've done a pretty good job about building our set piece as a forward pack."
Australia said this week they have been focussing on their defence against the free-flowing Fijians, and trying to prevent the Pacific Islanders from throwing offload passes in the tackle.
And that despite not having lost to Fiji since 1954.
"When we hear that, it gives us confidence," said Dewes.
"The main thing for us is that we just need to worry about ourselves and working on the basics, doing the basics really well and if we can do that they can practise all they want."
