The South Africans chased down Australia's mediocre 105/8 with 11 balls to spare to book their place in the final of the age-group tournament for the first time.
Australia elected to bat first but were put on the back foot immediately by Nthabiseng Nini, who trapped Ines Mckeon in front with the very first ball of the match, then affected a run-out in her next over, leaving Australia 12/2.
Captain Lucy Hamilton and Caoimhe Bray were measured in their rebuilding. Proteas Women captain Kayla Reyneke then dismissed her opposite number with a straight one that Hamilton missed, crashing into her pad.
The Proteas Women spinners then did a superb holding job, putting the strangle on the Australian batters. That set up Ashleigh van Wyk to rip through the middle order, bagging four wickets for just 17 runs as Australia limped to 105 off their allotment.
In reply, opener Jemma Bothma wasn't going to die wondering. The 17-year-old swatted five fours and two sixes on her way to a swashbuckling 37 off 24 to get the Proteas Women off to a superlative start. They lost Somine Lourens (5) and Fay Cowling (7) early, but Bothma continued to bludgeon the bowling attack.
Bothma eventually holed out in the deep off Hamilton's bowling, leaving the Proteas Women 56/3, though the damage had been done.
Captain Reyneke steadied the ship with Karabo Meso, taking South Africa to the brink. Both fell at the end but Mieke van Voorst and Seshnie Naidu combined to get the handful of runs required to get them into the final.
Proteas Women await either England or India in the final, which gets underway on Sunday.