The result saw Australia bounce back from a humiliating defeat at the hands of Zimbabwe at the weekend and assured them of a place in the triangular one-day series final.
But it could have been a different story had it not been for the big hitting of Marsh (86 not out), whose late flurry of runs helped Australia to a total of 282-7.
South Africa were lacklustre in their chasing of that target - with the exception of Faf du Plessis who hit 126 - and were all out for 220.
"If you look at the scoreboard now, it looks like we didn't compete," said De Villiers. "Having won the toss I thought we'd be in for a competitive game which wasn't the case.
"It felt good for the first 40 overs, the spinners pulled it back very well. Mitchell Marsh played an amazing knock at the end and showed that once you get in on this wicket it gets a lot easier.
"I don't think our bowlers were too bad. We've seen in the IPL and all around the world the batters getting very dangerous these days at the end, especially when they get a bit of freedom like that."
South Africa will now face Zimbabwe - over whom they hold a five-point advantage - in the final group stage fixture on Thursday.