The stand-in skipper, fresh from steering his side to a clean sweep of three ODI wins against Zimbabwe, will take the reins again while Cummins, fresh off the plane, concentrates on preparing for the upcoming Test series against the Proteas.
Asked to explain to anyone who might be confused about the division of leadership duties now that Cummins had arrived, Marsh, taking the eve-of-match press conference duties on Wednesday, Marsh explained: "Well, I'd imagine that he (Cummins) won't be playing tomorrow.
"I've said it multiple times now, but we work really well together. Pat's main focus is the Test group, and a lot of our bowlers and him in particular are preparing for that Test series.
"I'm hopeful that he'll feature in this series at some stage but while he's not, I'm very happy to do it. Myself, Pat, (coach) Andrew McDonald, and (chair of selectors) George (Bailey) work really well collectively, and that's how it happens."
Asked who would be skippering the side if they were both playing, Marsh smiled: "I guess we'll find out when when he plays."
Even without Cummins, Marsh has a vastly experienced attack, featuring Mitchell Starc, Josh Hazlewood and Adam Zampa, with a combined 351 ODIs under their belts, ready to spearhead the ODI World Cup reconnaissance mission against what looks, in contrast, a callow South Africa bowling attack.
With a year to go to the African-hosted World Cup, the latest three-match series feels more than just a warm-up for a fascinating first Test series in South Africa for eight years, but a chance to really get a feel for what shape the champions are in to defend their 50-over crown.
Disappointingly, though, the hosts, usually ready to unleash a hostile pace attack on any unsuspecting visitors, are really depleted, short of five injured quicks who would have been expected to provide a real test -- Kagiso Rabada, Lungi Ngidi, Kwena Maphaka, Ottneil Baartman and Nandre Burger.
Lizaad Williams, twins Marco and Duan Jansen, Gerald Coetzee, Corbin Bosch and Nqobani Mokoena have, in contrast, played just 72 ODIs between them.
"I think we're going to have to make the most of the bowling resources that we have from the seam department. There are options there. I think it's just how smartly we approach managing the loads of our fast bowlers," skipper Temba Bavuma told reporters on Wednesday.
"We want teams, when they come here, to know that they're going to have to deal with our fast bowlers. That's part of our DNA."
So Australia will, naturally, expect pace, but as Starc noted, the unusual time of year for this visit with rain potentially a major factor means "there's an element of the unknown.
"It's quite muggy and humid. There has been talk about the rain before we got over here. It's probably not that typical dry part of the season just yet.
"It could be interesting to see what the weather does over the next couple of days and then for the game as well."
The evergreen 36-year-old had plenty of praise for the in-form Nathan Ellis, who led the Zimbabwe sweep with a combined 9-112.
"He is really pushing his case as a lock as a pick in the one-day team, and no doubt he'll look to perform in these three as well," said Starc.
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