If pool rivals Fiji score four tries en route to a bonus point win over Georgia in Bordeaux on Saturday (Sunday AEST), Australia's slim hopes of reaching the quarter-finals will be crushed.
Stinging from their record loss to Wales last round in Lyon, the Wallabies will look to salvage something from the disastrous campaign under coach Eddie Jones, who has been roundly blamed for the debacle.
They will chase five points if they are alive but, regardless, they're determined to make a statement against world No.16 Portugal on Sunday (Monday AEST) in Saint-Etienne, with a loss unthinkable for the two-time world champions.
Australia will again be without skipper Will Skelton (calf) and star prop Taniela Tupou (hamstring).
Five-eighth Ben Donaldson said the battered players had regrouped following their second pool loss to Wales which has all but ended their campaign.
"Losing with that scoreline in a World Cup, it's obviously very devastating," Donaldson said.
"We've got another Test match this week so that's the best way to get over a result like that ... to put in a good performance for ourselves, our coaches and our fans here and also back at home.
"We've got so much support and that's one of the reasons why everyone was so upset, because we've let the whole country down.
"The boys are already moving on and we had a great session today so we will be ready to go.
"It's pretty easy to get up for a Test match as you never know when it could be your last so we're just trying to put everything out there on the line."
An Australia A line-up took on the Portuguese in a World Cup warm-up match in France last month, needing to overturn a 12-14-half-time deficit to post a 30-17 victory.
Portugal fell to Wales by a smaller margin than the Wallabies and drew with Georgia so will be far from easybeats, according to Donaldson.
"They pushed Wales for a good 50-60 minutes and they probably should have won last week against Georgia as well so they're a growing team," the 24-year-old said.
"They're playing some real expansive footy, they've got some electrifying players in their team so we need to be on our game this week."
Donaldson said the Wallabies needed to get off to a confidence-boosting start to put Portugal on the back foot.
"They like to play off unstructured ball so if we come out and we're being too loose and we give them a bit of momentum to start with, it's going to be pretty hard to gather that back.
"We just need to be clinical, stick to our game plan as we've got a good plan in place.
"The coaches have been unreal this week, so just come out fast and be physical and hopefully the game rolls on from that."
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