Harris-Tavita was knocked out cold in the last World Cup final four years ago and is now long-odds to play again for Samoa this campaign.
The Warriors' No.6 strained his hamstring while taking a shot at goal in Sunday's 20-18 group stage loss to Tonga.
Parramatta half Volkman made an instant impact, scoring a try after replacing Harris-Tavita from the bench, and he will step in against the Kangaroos.
Volkman's rise to a likely Samoan starting jersey caps a terrific season for the 24-year-old after he was dumped by St George Illawarra in 2024 with concerns over a long-term shoulder issue.
Samoa face Australia on Sunday at Sydney's Allianz Stadium after their narrow loss to the Tongans dumped England out of the tournament on points differential.
"Ronald's done a great job, he did a fantastic job last week for us in the win over Papua New Guinea," Samoa coach Ben Gardiner said.
"He came on at the back end of the game, in a slightly different role for a different reason and he did a really good job when he came on.
"I thought he brought great energy, I thought his speed in the line really put pressure on the Tongan team.
"We've got trust in every player in our team, so it doesn't matter who goes on.
"The guys that are in our squad, we've got complete trust in them."
The 24-year-old Volkman is likely to be the target of heavy traffic from the Kangaroos' edge forwards as the two sides renew a rivalry that has simmered since the World Cup final four years ago.
Samoa have yet to beat Australia in a Test across fives meetings and lost 38-12 in their last international three years ago.
"Their form's been very strong, very strong and they've got fantastic players," Gardiner said.
"They've got players that have played a lot of games for the Kangaroos and are in a really good position at the moment.
"They've won three footy matches on the trot, and they're going to be a really hard side to beat next week.
"If we hold on to the ball and we complete better than 66 per cent that'll give us a good opportunity to pick up on some of those things we need to work on."
