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 team
Starting XV: 15 Tom Wright; 14 Andrew Kellaway, 13 Len Ikitau, 12 Hunter Paisami, 11 Marika Koroibete; 10 Noah Lolesio, 9 Nic White; 8 Harry Wilson, 7 Carlo Tizzano, 6 Rob Valetini; 5 Lukhan Salakaia-Loto, 4 Angus Blyth; 3 Allan Alaalatoa (captain), 2 Josh Nasser, 1 Angus Bell
Replacements: 16 Billy Pollard, 17 James Slipper, 18 Zane Nonggorr, 19 Tom Hooper, 20 Seru Uru, 21 Tate McDermott, 22 Ben Donaldson, 23 Max Jorgensen
South Africa team
Starting XV: 15 Aphelele Fassi; 14 Cheslin Kolbe, 13 Jesse Kriel, 12 Lukhanyo Am, 11 Makazole Mapimpi; 10 Sacha Feinberg-Mngomezulu, 9 Morne van den Berg; 8 Elrigh Louw, 7 Pieter-Steph du Toit, 6 Marco van Staden; 5 Ruan Nortje, 4 Salmaan Moerat (captain); 3 Thomas du Toit, 2 Johan Grobbelaar, 1 Jan-Hendrik Wessels
Replacements: 16 Malcolm Marx, 17 Ox Nche, 18 Vincent Koch, 19 Eben Etzebeth, 20 Kwagga Smith, 21 Grant Williams, 22 Manie Libbok, 23 Handre Pollard
