Mitch Marsh has continued his dream run as Australia's Twenty20 captain, punishing South Africa's bowlers for the second time in as many games to rocket his side to a series-clinching eight-wicket win at Durban.

After thumping an unbeaten 92 in his captaincy debut in the first game, Marsh produced an equally swashbuckling encore, thumping an unbeaten 76 off 39 balls at Kingsmead on Saturday morning AEST.

His innings helped Australia chase down 165 with 31 balls to spare, and secure an unassailable 2-0 series lead.

The Proteas have been powerless to find any answers for Marsh, who thumped six sixes in his thrilling knock and has racked up 168 runs off 86 balls for the series without being dismissed.

He had a match-winning partnership with Matt Short who followed his promising 20 on debut in the series opener with 66 off 30 deliveries, which included four sixes.

After Travis Head departed for 18, Short's clean striking and Marsh's brute power paved the path for a second successive belting.

Short crunched Lungi Ngidi's second over for 20 to close the powerplay strongly before Marsh smashed spinner Bjorn Fortuin's first over for 18.

Marsh offered one tough chance on 32, Tavemba Bavuma almost pulling off a screamer in the outfield.

Marsh and Short combined for 100 runs — Australia's best second-wicket partnership against South Africa — before Short sliced spinner Tabraiz Shamsi to Aiden Markram.

Marsh went to another level of aggression following Short's exit, swinging hard to the finish.

Earlier, South Africa captain Aiden Markram's 49 helped the Proteas to a respectable 8-164 after their top order had been blown away.

Opener Temba Bavuma thumped 35 of South Africa's first 36 runs but his departure sparked a 4-10 collapse from the last three overs of the powerplay.

Seven of those runs came from a no-ball and free hit six thumped by Rassie van der Dussen off Sean Abbott.

Markram and wicketkeeper Tristan Stubbs (27) joined forces at 4-46 and put on 51 for the fifth wicket to give their attack something to bowl at.

Seamers Sean Abbott (3-22), Nathan Ellis (3-25) and Jason Behrendorff (2-25) all managed to conjure appreciable sideways and manufacture regular breakthroughs before Marsh and Short provided the real fireworks.

AAP/ABC