Ikitau touched down twice in the first half although the Springboks took the lead through centre Lukhanyo Am two minutes into the second half.
Koroibete struck twice in six minutes to seal a bonus-point Australia win and also regained the Nelson Mandela Trophy.
Ikitau scored the opening try and crossed for a second time in the 21st minute the Springboks trimmed the deficit to three points by half time.
The visitors quickly snatched the lead after the restart when Am touched down. However, Australia regained the initiative thanks to winger Koroibete, whose first try was created by Taniela Tupou’s audacious no-look pass.
Koroibete’s second established a 13-point lead and Australia defended it stoutly, repelling waves of South African attacks in the closing stages to see out a victory. (BBC)