Dale Steyn ripped the heart out of Australia's batting and Faf du Plessis came within a whisker of yet another century as South Africa cruised to a six-wicket victory to clinch the triangular series cricket trophy.

Steyn swung the ball at pace to rip out three quick wickets and record figures of 4 for 35 as Australia were limited to 217 for nine.

Du Plessis then fell four runs short of what would have been his fourth hundred of the series as the Proteas chased down their target with 55 balls to spare in front of a packed Harare Sports Club.

Although Du Plessis was the obvious man of the series for his 464 runs in five innings, Steyn was the clear man of the match.

The South African quick made the early breakthrough when he had Phil Hughes brilliantly caught by AB de Villiers, but his most significant contribution came when he swung the ball at pace to pick up three wickets in the middle period.

"I think they bowled very, very well," Australia captain George Bailey said. "It's actually easier to swallow when you can say that you were outplayed.

"I thought they bowled really tight and made it hard for us to score early, and then once they got the ball reversing, Dale Steyn was able to show his class."

Steyn bowled top scorer Aaron Finch for 54 with a vicious in ducker, then repeated the delivery to trap Glenn Maxwell lbw with the very next ball.

Brad Haddin was also trapped in front at the start of Steyn's next over, and with Wayne Parnell and Morne Morkel also striking Australia were reduced to 144 for eight in the 38th over.

"He's the best bowler of reverse swing in the world by a long way," South Africa captain AB de Villiers said of Steyn.

"I think he showed more skills than most of our bowlers and the Australian bowlers."

It took a 71-run stand for the eighth wicket between James Faulkner and Mitchell Starc to drag Australia towards some sense of respectability, but their total always looked well short.

Although Quinton de Kock fell to Maxwell early in the reply, Hashim Amla stroked 51 as he added 98 for the second wicket with Du Plessis.

The partnership was ended by leg spinner Steve Smith, and an experiment of promoting Wayne Parnell to No. 4 failed when he was bowled by Faulkner, but Du Plessis and De Villiers got South Africa back on track with a 91-run partnership.

That was only broken when the scores were level as Du Plessis attempted to hit the boundary that would have made him the first player to score four hundreds in an ODI series, and instead found mid-on.

However De Villiers finished the match the very next ball with a crushing drive through extra cover for four to finish unbeaten on 57 from just 41 deliveries.

"There's always been doubt on us in big-pressure games, and I guess we showed what we're capable of today," De Villiers said.

"We all feel like we're on the right track as a team and I think we've got a good idea of where we're heading after this series."