The four-test series resumes in Cape Town from March 22 and Australia will be glad for the extended break as all-rounder Marsh races to recover from a groin strain.
However, even if Marsh is fit for the third Test, there is still the risk of him aggravating the injury.
"Mitchell Marsh probably deserved a hundred in the first test to be fair, but if we're getting batters scoring big hundreds it certainly helps us out a hell of a lot".
Australia lost the second Test by six wickets in Port Elizabeth on Monday to leave the four-match series tied at 1-1.
Peter Handscomb is the reserve batsman in the squad but selectors may opt to summon a like-for-like cover player for Marsh, such as Marcus Stoinis.
The 26-year-old has become Australia's middle-order rock since his Ashes recall, with David Warner the only Australian to have scored more runs than him in the current series.
"He's got a bit of a groin strain so he was struggling to move a little bit", Smith was quoted as saying by ESPNcricinfo.
"All the quicks are going really well", he said.
He put down a simple catch when South Africa were 0-7. "I was told at the break that he can actually run, so unfortunately he could've been out on the field when he was in the slips, but still should've taken that catch but everyone drops it every now and again, I've been pretty guilty myself in recent times".
"In regards to Mitch Marsh, it's a good break now between now and the next test match and he's telling me he'll be fine but we'll wait and see how things go", said Smith, who also gave a clean bill of health to Australia's pace trio of Mitchell Starc, Josh Hazlewood and Pat Cummins. "I haven't heard (otherwise) today, but before today they were all feeling fine", he said.