“Lot of emotions in the room right now. Haven’t been at our best. Disappointed bunch at the moment.”
Nissanka was involved in a 97-run stand off 66 balls with Kusal Mendis for the second wicket, with the wicketkeeper-batsman posting his third 50 in the competition.
Nissanka became the first Sri Lankan to score two hundreds in T20 internationals.
After Mendis’ dismissal, Nissanka and Pavan Rathnayake put on 76 off 34 balls to see Sri Lanka to victory.
Nissanka played some glorious cover drives off the spinners and played some elegant flicks against the quicks.
Sri Lanka came into the World Cup after a 3-0 drubbing at the hands of England, but have gained momentum and have won all three games so far.
Australia’s campaign has been plagued by injuries.
Marsh, who missed the first two games, returned and gave them a rollicking start by scoring 54.
Marsh and Travis Head (56) scored 104 runs off 51 balls for the first wicket.
With Australia eyeing a total in excess of 200, Sri Lanka’s slow bowlers kept them to 181 with leg-spinner Dushan Hemantha picking up 3-37.
Australia collapsed as they looked for quick runs, losing their last six wickets for 21 in 24 balls.
In Group C, England also booked a spot in the Super 8 stage, with West Indies in Group C, after a 24-run win over Italy.
The Black Caps can also reach the next round with a win over Canada tonight.