The 30-year-old de Kock, who will be retiring after the World Cup, followed up his hundred against Sri Lanka with another masterful 109 off 106 balls on an untested wicket in South Africa’s total of 311-7.
Australia, which won the toss and elected to bowl on a relaid pitch, were sloppy on the field, dropping five catches before getting bowled out for 177 — their second below-200 total in the tournament.
Fast bowler Kagiso Rabada picked up 3-33 and Keshav Maharaj clamped the batters by chipping in 2-30 in a 10-over spell.
Only Glenn Maxwell (2-34) showed some spark with his offspin bowling in an otherwise poor day on the field for Australia, which had been seend as one of favourites to reach the semifinals.
South Africa climbed on top of the points table with two wins from two games while Australia are without a point after losing both their opening games.