Australia took a 1-0 lead in the three-match series after posting 226-6 in their 20 overs, the highest score at Kingsmead, before restricting South Africa to 115 all out in a reply that never really got going.
The second match in the series is on Friday before it concludes two days later. All three games will be played in Durban. Both sides are missing several of their regulars.
After being sent in to bat, Australia’s innings was anchored by Marsh, who reached his 50 in 22 deliveries.
He put on 97 in 50 balls for the fifth wicket with Tim David, who had no trouble clearing the small boundaries with four sixes on his way to 64 from 28 balls.
"It's nice to contribute for the team," Marsh said at the post-match presentation. "I thought Tim David was fantastic with the way he took pressure off me."
South Africa were sloppy in the field and at times battled to control line and length with the ball. Seamer Lizaad Williams was the pick of the home attack with 3-44 in his four overs.
Opener Reeza Hendricks led a near lone challenge to Australia’s total with a well-constructed 56 from 43 balls, as the home side were pegged back by an excellent spell of 4-31 from debutant leg-spinner Tanveer Sangha.
"Sangha has done well in the Big Bash and he's a fantastic talent," Marsh said.
Seamer Marcus Stoinis pitched in with 3-18 in a fine display of swing bowling.
Australia also handed debuts to opening batter Matthew Short all-rounder Aaron Hardie and fast bowler Spencer Johnson, while South Africa’s under-strength side included new faces in batter Dewald Brevis and seamer Gerald Coetzee.
"The wicket played really well and there was a bit of rust in every discipline," South Africa captain Aiden Markram said. "We have to come back strong on Friday."
