The batting bias of cricket's shortest format is evident in the ongoing Indian Premier League (IPL), where 200-plus scores have become a routine and 300 does not look impossible anymore
Steyn took key wickets in recording figures of 4-34 as the Proteas limited Australia to 217-9 in their 50 overs.
South Africa always looked in control of their reply as Du Plessis finished with 96, just missing out on his fourth century in five matches and third in the series against Australia.
The Proteas passed their victory target with 55 balls to spare to win the one-day international series that also involved Zimbabwe.
Both sides had spoken of the importance of this series in the build-up to next year's 50-over World Cup in Australia and New Zealand.