It is their fourth victory of the tournament as they jump to top spot in the standings with eight points.
Chasing 338 to win, Sri Lanka started the run chase well but a collapse in the middle order, From 186-3, Sri Lanka lost 7 wickets for 61 runs and were bundled out for 247.
Angelo Mathews (09) , Milinda Siriwardana (03) and Thisara Perera (07) departed cheaply. Kusal Mendis also fell soon thereafter, holing out to David Warner off Starc.
Mitchell Starc (4/55) and Kane Richardson (3/47) were wrecker-in-chiefs for the Aussies, while captain Dimuth Karunaratne top-scored for Sri Lanka with a well-made 97 off 108 balls.