In next Wednesday's opening match in the South African city of Durban, Marsh will become Australia's 12th men's Twenty20 skipper, as officials ponder who will replace previous captain Aaron Finch on a full-time basis.
Marsh will also lead the one-day international (ODI) side against South Africa, with Pat Cummins no certainty to return from a wrist injury for the start of the five-match series.
Cummins will lead Australia to the ODI World Cup in India in October and November, but has not committed to retaining the captaincy beyond that.
If selectors opt to merge the roles and have one man in charge of both white-ball sides, as Finch was before his retirement, Marsh is now the clear favourite.
He has captaincy experience, including taking over as Western Australia's leader at the age of 25.
The now 31-year-old last September ruled himself out of the race to become Australia's white-ball captain, in a bid to focus on his own game.
But speaking for the first time since his appointment earlier this month, Marsh said his relationship with Cummins and coach Andrew McDonald had put him at ease with the role.
"At that time I probably just didn't want to be spoken about," Marsh said on Tuesday, reflecting on his withdrawal from consideration last year.
"It honestly comes back to the relationship I have with Andrew and Patty, working together as a team.
"I've come a long way as a person and learnt a lot captaining Western Australia, having great people like (coach) Adam Voges and Ashton Turner around me to learn as much as I can.
"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."
Australia's next T20 captain will lead the country to 2024's World Cup in the Caribbean and USA, with the following showpiece ODI tournament in four years' time.
Marsh said he was now open to taking on the job full time.
"I'm available to captain for as long as the team needs me. We'll see where it all ends," he said, adding he was looking forward to working with Cummins when the quick takes back the captaincy for the World Cup.
"That's probably one of Pat's strengths, he leans on other people in the squad and other leaders," Marsh said.
"There's no doubt, with his role as a bowler, playing every game in every format is near-on impossible. So for him to have guys he trusts and leans on (is crucial).
"We have a great friendship, first and foremost, and a great relationship professionally. I always know I can lean on him and vice-versa."
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