Vice-captain McDermott missed the Fiji match with concussion, while the hulking Fa'amausili, who has been sidelined by a calf injury since the team arrived in France, is expected to replace the injured Taniela Tupou against Wales.
"It adds a lot, a big, ball-carrying tighthead prop and somebody who's captained a team," Wallabies assistant coach Neal Hatley told reporters on Wednesday.
"Really important players for us and to get people coming back into the mix is always fantastic. It's great to have them both back and we're thrilled about it."
It was not lost on Hatley that McDermott captained the side and Fa'amausili played a blinder when Australia took a 17-3 lead against the All Blacks in a narrow loss in Dunedin last month, their best 40 minutes of rugby this year.
McDermott said he did not know yet whether he would lead the side out in Lyon in place of Will Skelton, who has been sidelined by a calf injury.
"We've got a strong leadership group," he said.
"So anyone in that group is capable of taking that role. You saw Dave Porecki take over for (Fiji) and I missed that game. So we'll just have to see."
McDermott said he expected Wales, who won their first two matches and top Pool C, to target the Wallabies at the breakdown after Fiji secured 11 turnovers in Saint Etienne.
"They'll definitely go to the breakdown considering what went on at the weekend, there's no questions about it. They're going to come hard there," he added.
"They've got good backrowers that will put pressure on us there. We've got to move the point of attack. We've got to win that physicality and that mindset battle at the breakdown in order to do that."
The twice world champions face a first-ever pool-stage exit from the World Cup if they cannot beat Wales on Sunday and McDermott said that had sharpened the players' appetite for the contest.
"We have to get that result," McDermott said. "So we've had a big shift from the hangover from that performance against Fiji.
"We had to throw that in the bin as quickly as possible because of the importance of this game."