South Africa beat Australia for the third game in a row when they won the second Twenty20 international by 17 runs on Sunday.
After clinching the Test series by winning the first two matches, Australia lost the third Test in Cape Town by an innings and the opening Twenty20 match by four wickets.
Chasing 157 for victory on Sunday, Australia were derailed by off-spinner Johan Botha, who claimed two for 16 in four overs as the visitors crashed to 103 for five after 15 overs having been 63 for two at the halfway mark.
Earlier, debutant Roelof van der Merwe lifted South Africa to a respectable 156 for five with a lively 48 off 30 balls.
Van der Merwe, playing on his home ground, then picked up the key wicket of David Hussey for 27 as South Africa's spinners took control, restricting Australia to 139 for eight.
Australia will hope for a prompt return to form when the five-match one-day international series gets underway in Durban on Friday.