Bulls number eight Elrigh Louw will also line up for the first time from the off at the Suncorp Stadium on Saturday in a side that otherwise relies on the core of players that won last year's World Cup and drew a two-Test series with Ireland last month.
Both Feinberg-Mngomezulu, 22, and Louw, 24, have made four previous appearances as replacements for the Boks.
"Sacha has made a notable impact off the bench in all four Tests he has played so far, and he's slotted into the system with ease, so we are excited to see what dynamic he adds to the team as a starter," said Erasmus.
"His goal-kicking has also been impressive, and we know that against teams like Australia, New Zealand, and Argentina the margins are very small, so every point will count.
"We’re also keen to see what Elrigh brings to the game, given his consistently strong performances for the Bulls in the last few seasons.
"With Pieter-Steph (du Toit) and Siya (Kolisi) alongside him, he’ll have good guidance to play to his strengths and do what he does best."
The matchday 23 features just three changes from the group that faced the Irish with Louw, two-time World Cup-winning scrumhalf Cobus Reinach, and replacement utility forward Ben-Jason Dixon the only additions.
Regular scrumhalf Faf de Klerk is nursing a niggle while the experienced Pollard is just one of two specialist backs on the bench along with Grant Williams, who serves as back-up at scrumhalf and wing.
“One of the big talking points for us as a team from the beginning of the season was that we need to get the balance right between selecting teams featuring youth and experience as we build towards the 2027 Rugby World Cup," said Erasmus.
"We’re pleased with the way we’ve been able to integrate some of the young players into the squad this season and give them game time against top opposition.
“We’re excited to see what they have to offer against a team such as Australia."
South Africa go into the match as double World Cup winners but they face an Australia side that has won its last three Tests in a city where the Boks have not won since 2013.
"They have a good track record against us in the last few years, so we know it’s going to be a massive challenge," said Erasmus.
South Africa team (15-1): Willie le Roux; Cheslin Kolbe, Jesse Kriel, Damian de Allende, Kurt-Lee Arendse; Sacha Feinberg-Mngomezulu, Cobus Reinach; Elrigh Louw, Pieter-Steph du Toit, Siya Kolisi (captain), RG Snyman, Eben Etzebeth, Frans Malherbe, Bongi Mbonambi, Ox Nche
Replacements: Malcolm Marx, Gerhard Steenekamp, Vincent Koch, Ben-Jason Dixon, Marco van Staden, Kwagga Smith, Grant Williams, Handre Pollard
