The see-saw contest at the Eden Gardens turned out to be a tale of two powerplays and Australia dominated both en route to a nervy three-wicket victory that sealed their place in Sunday's final against India.

South Africa were 18-2 in 10 overs after electing to bat in what was the lowest powerplay score in this year's tournament.

They had to wait until the ninth over for their first boundary and they slumped to 24-4 inside 12 overs as the pressure mounted on them.

Having bowled out South Africa for 212, Australia cruised to 74-2 after 10 overs with Travis Head (62) and David Warner (29) plundering 60 runs from the first six overs.

The flying start meant Australia could afford to go slow after losing quick wickets towards the back end of their innings.

"We wanted to hit hard with the ball," Starc said of their bowling approach against South Africa's top order that includes David Miller, whose gallant 101 went in vain, and Heinrich Klaasen.

"We know that South Africa are very strong at the back end if they've got wickets in hand and (we knew) if we can get ) Miller and Klaasen early or within the first 20 overs, then we're well on our way."

When Australia came out to bat, their openers Head and Warner had a similar aggressive approach.

"The way we set up with the bat is to really be aggressive, take the game on and the freedom to give those guys to go out and play the way they like to," Starc said.

"The way we started with the bat meant we didn't have to go as hard at the back end.

"We've seen throughout the tournament how tough the first 10 overs can be at certain times of the day.

"Sometimes you take a little bit of luck with you but the guys were fantastic in that opening partnership and then Trav carried through a bit longer."