Marsh is stepping in for injured one-day captain Pat Cummins while Cricket Australia has yet to decide on a T20 skipper for the long term following the retirement of Aaron Finch.
Being named captain of the limited overs sides continues a memorable year for Marsh, who got married in April and scored a century on his recall to the test squad during the Ashes.
"It's certainly been a crazy year. Throw in getting married, it's been a good one," he told reporters in Sydney.
"We’ll get through South Africa and see how we go.
"I’m available to captain for as long as the team needs me. We’ll see where it all ends."
Cummins, who also captains the test side, is expected to link up with the squad in India ahead of the 50-overs World Cup which starts in October.
Marsh said he would probably not "try to reinvent the wheel" while captaining Australia for three one-dayers and five T20s in South Africa.
"I think the most important thing I’ve learnt about leadership is staying true to yourself," he said.
"Hopefully I’m a good captain. If not, I won’t be in the job for very long. That’s OK. I’m just looking forward to the opportunity."
Marsh is looking forward to seeing left-arm pace bowler Spencer Johnson push for a potential debut in place of injured regular quick Mitchell Starc.
Young legspinner Tanveer Sangha will also hope for a debut.
"Think there's no doubt we are going to see them in Australian colours over the next few years and we'll get a look at them in South Africa," he said.
Steve Smith is among the tour's absentees while recovering from a wrist injury which he carried during the Ashes.
Smith on Tuesday revealed he had torn a tendon in his left wrist during the second test at Lord's, which had ended his hopes of trying his hand at opening in the T20 series in South Africa.
"It’s kind of the dream job. Everyone wants to open the batting in T20s," he told Australian broadcaster Fox Sports.
"There’s not much accountability there, you just sort of go out there and play."
