India captain Rohit Sharma led from the front with a blistering 92 – from 41 balls, including seven fours and eight sixes – as his side posted a total of 205-5.
Australia were then held to 181-7, opener Travis Head top-scoring with 76 in an innings where left-arm paceman Arshdeep Singh took 3-37.
India will now play defending champions England in Thursday’s semi-final in Guyana.
Australia will fail to reach the last four if Afghanistan, who defeated them by 21 runs in a stunning upset last time out, beat Bangladesh in the concluding Group 1 game of the Super Eights stage (2:30am SA time).
The Proteas, unbeaten in the tournament, have qualified alongside England for the semi-finals out of Group 2.
Photo: Gareth Copley/Getty Images
