From what I read a while back, so I'm paraphrasing, is that the government wanted all companies, either state owned and private, that supplied them to be as ethnically diverse as the country. That's roughly 1:5 white to black, again roughly speaking but it may be somewhat different.
Engineers working for the water company where predominantly white, almost all of them at the time this came into effect. so what the water company did was fire enough of the white engineers for this ratio to be adjusted, then they couldn't find enough black water engineers to fill the jobs. Instead of filling the jobs to keep the company running, they've left the jobs open still looking for these black engineeers.
I can't find the exact story I read at teh time, but this one talk about the same issue:
https://www.news24.com/SouthAfrica/News/Eskom-must-shed-1-081-white-engineers-20150308
Also the nation government meddling with projects that would have sorted this years back didn't help either.
Here's a story about the affirmative action targets directly:
https://mywage.co.za/decent-work/fair-treatment/affirmative-action