Koroibete, 32, will start in Perth to play his first Test since last year's World Cup, replacing Filipo Daugunu who suffered an injury in last week's 33-7 thrashing by the Springboks in Brisbane.
"The players have rolled up their sleeves and worked hard again this week," Schmidt said in a statement.
"The reality is that we need to be more combative and accurate this Saturday."
Schmidt has also included experienced scrum-half Nic White for Jake Gordon, who drops out of the matchday squad with Tate McDermott on the bench.
Loosehead prop Angus Bell returns from injury, alongside his former junior international teammate Josh Nasser, who earns his first Test start at hooker, while Allan Alaalatoa captains the side from tighthead prop.
Lock Angus Blyth replaces Nick Frost in the second row alongside Lukhan Salakaia-Loto.
Uncapped back Max Jorgensen and flanker Seru Uru could both make their debuts off the bench.
Veteran prop James Slipper could make his 138th Test appearance off the bench, leaving him one short of George Gregan's record for Australia of 139.
South Africa have made ten changes with scrum-half Morne van den Berg and lock Ruan Nortje making their first starts for the Springboks.
Australia make five changes for second Springboks Test
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