After Travis Head's century and Scott Boland's defence initially repelled the Proteas early on Thursday's final day, Australia lost their last wicket with 28.5 overs left at St George's Park.
The Proteas' victory has levelled the battle between the world's two top-ranked sides at 1-1, setting up a mouth-watering series decider in Cape Town starting on Tuesday.
And it is clear that Australia must be much better in that contest, after one of the most thumping defeats of the Cummins captaincy era.
Aided by a washed-out morning session, the tourists did well to hold on for as long as they did with Boland soaking up 166 balls as nightwatchman.
But after Head chopped on for 121 and Steve Smith played a poor short to be caught pulling Kagiso Rabada on three before tea, Australia's fate was sealed.
Boland fought on before he was bowled by Wiaan Milder (3-64) after tea, leaving Alex Carey (37no) running out of partners.
And when Kagiso Rabada returned to go through Mitchell Starc before Josh Hazlewood guided Marco Jansen to the slips, the game was over.
Ultimately, Australia have been left to lick their wounds in a match they should never have been in a position to lose.
Cummins' men had South Africa 6-110 early on the third morning, after Boland's five-wicket haul ripped through the Proteas on a wet opening two days.
But they allowed man-of-the-match Temba Bavuma to lead a South African recovery with a real captain's knock before declaring at 9-425.
And when Australia collapsed to be out for 99 on Wednesday and were asked to follow on, they were fighting to save the match.
Australia's total was their lowest in a first innings in almost a decade, but as Cummins pointed out, it was hardly a one-off.
The tourists were 6-124 themselves in Durban last week, while they were also bowled out for 198 by Bangladesh in a shock home defeat in August.
"Even last week (Matt) Renshaw had an amazing game, but I still think we weren't at our best, particularly in batting," Cummins admitted.
"Looking around the changeroom and how good our batters are and the standards they hold themselves to, we're probably a little short of the runs we'd like to be.
"We're still finding ways to win.
"We make it hard for ourselves sometimes.
"We've got three or four days to get it right for Cape Town. There'll be lots of conversations."
Australia's batting was labelled "gutless" by former Proteas' quick Vernon Philander during their collapse, as they lost 13 wickets in a day for the first time since 2011.
"It depends on what that means really. I think you've got to find a way in Test cricket," Cummins said when asked if the accusation was fair.
"It's not always going to be easy.
"You saw today with Trav and Scotty, sometimes it's going to be about getting through the tough spells and sticking to your method.
"I think we went away from that on day four. We had a couple of loose shots, and could have hung around and waited for the game to come to us."
This Test will go down as one of the Proteas' great wins given the position they were in.
For Australia, there will be recriminations, with Cummins admitting selectors needed to decide between offering more time to struggling players or pulling the pin.
Marnus Labuschagne, Nic Maddinson and Beau Webster are all in the spotlight, particularly with Cameron Green and Josh Inglis fit again.
Labuschagne's shot to get out, taking spinner Simon Harmer on into gusty winds, has already drawn criticism.
Smith's wasn't much better against Rabada on day five, given the match situation.
The same could be said of Starc's big attempted drive at the Proteas' quick when he was bowled as dark clouds hovered overhead and briefly threatened play.
He punched his bat in frustration as he walked off, but he has the excuse of being a lower-order batter.
Instead, it is the established batters who must accept the blame for Australia's crushing defeat.
16°C