Brooke Halliday made 45 from 37 balls and, alongside Devine, had New Zealand set at 186 for three in the 38th over.
But left-arm spinner Mlaba, who finished with 4-40, removed both Devine and Halliday as the New Zealand lower order collapsed for the second straight game, losing their last five wickets for 36 runs.
In South Africa’s chase, Jess Kerr struck in the third over, removing captain Laura Wolvaardt after she’d started with three quick boundaries. After the initial appeal for LBW was turned down, New Zealand referred the decision, which was overturned.
However, that brought Brits and Luus together and across the following 28 overs they put South Africa in a commanding position before Brits was eventually bowled by a Lea Tahuhu slower ball, having just brought up her fifth century of the year – the most by a woman.
The damage was done by South Africa and they cruised to their first win of the tournament from there, giving their net run-rate a welcome boost after an opening game thrashing to England.
For New Zealand to make the semifinals, they can probably afford to lose just one of their remaining five games. The good news is they face Bangladesh, Sri Lanka and Pakistan next, all teams below them on the world rankings, before the two tougher games against India and England.
“Got into a good position to launch but kept losing wickets,” Devine said after the loss.
“Credit to the way they batted, they negated what we threw at them. Disappointing for us. We have spoken about it before. We have to win a lot of games. Puts a bit more pressure on the remaining games. We know exactly where we stand and what we have to do.
“Some things that went well in both innings but in this competition, you cannot play half a game.”
The White Ferns depart Indore for Guwahati, in Assam where they face Bangladesh on Friday.