South Africa's skipper will start at the base of the scrum for the first time in his career this weekend, shifting from his usual position of flanker.
Kolisi did feature in the eight role for the Sharks during their United Rugby Championship campaign and in fact played the majority of his domestic rugby there in 2024/25.
Erasmus has now decided to repeat that tactic at Test level with suspension ruling out Jasper Wiese, while Cameron Hanekom and Elrigh Louw are long-term absentees.
Jean-Luc du Preez could have filled in, having started against the Barbarians in June, and usual lock Cobus Wiese did a fine job against Georgia, but the Boks boss has instead opted for his captain, which makes room for Marko van Staden to join the loose trio.
According to Erasmus, the skipper's actual job will change little with the loose trio of Kolisi, Pieter-Steph du Toit and Van Staden selected for a specific reason.
"We actually changed our mind there. The more we practised we realised that the only position the guy plays at eight is at scrums which happens maybe 13 or 14 times a game," he told reporters.
"General play is probably what happens most and having guys like Pieter, Marco and Siya on the field we really feel Australia is a threat at the breakdown. They really fight hard at the breakdown, and those guys clean out well at the breakdown and we feel we are going to need that.
"It's just at scrumming where he is eight. I know I said a couple of weeks ago that Siya is a six and not an eight, but we came to the conclusion that we actually need him there and have Kwagga (Smith) on the bench."
As well as the decision to move Kolisi, there were other notable calls with Grant Williams and Manie Libbok selected in the playmaking positions, and Erasmus not picking a 6-2 or 7-1 bench.
The Springboks have instead gone with the traditional 5-3, but the head coach suggested that it may change depending on who the Wallabies select.
South Africa chose to move their squad announcement forward a day from Tuesday and the Bok boss explained why.
"It is to get the speculation out of the way," he said. "You guys can now speculate about the team that are playing.
"The team knows already who is playing so getting this out of the way we can just focus on really training well and preparing.
"If you look at the split of their squad there is a lot of forwards in there. If they go 6-2 we might change on Thursday or Friday but if they go standard 5-3 this will pretty much be our team."
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