Australia take on their hosts in the first match in Bloemfontein on Thursday where the wicket is expected to be flat like those in India but with the players having to deal with cold temperatures.
Australia have already selected their side for the opening game of the series with Marsh dropping down the top order to No. 3, behind Travis Head and David Warner, having opened with Head in Australia's last ODI which was a 21-run win over India in Chennai in March.
"There'll be potentially a few moving parts with our squad throughout these five games, I'm guessing it’s the same with South Africa with a heavy schedule, building to the World Cup. We'll have certain guys in different areas, but I'll start at No. 3 tomorrow," he told a press conference on Wednesday.
"I’m not really sure about the batting order past this game. It's really important that we focus on this one and try and win the first of the five-match series. It's very important as we build towards the World Cup, that we try and win this series first. That's number one on the agenda," he added.
Marsh captained Australia to a convincing 3-0 whitewash of their hosts in last week's T20 series but is expecting a much tougher challenge in the 50-over version.
"It was obviously a really pleasing win for us as a team, even as a pretty young team to play the way we did. South Africa welcome back a lot of experienced players now, so it'll be a different challenge for us and they play very well at home," Marsh said.
"It's going to be two really good teams going at it, but our mentality won't change. How can we play a nice attacking brand of cricket and I'm sure they will, too. It'll be a great series."
Australia's preparations for the World Cup have been hit by injuries to test captain Pat Cummins and Steve Smith, who both picked up wrist injuries during the Ashes series, ruling them out of the ODI series in South Africa. Mitchell Starc has also been sidelined by a groin problem.
