I'd call it possible but highly unlikely, of course. The idea that South Africa could erupt in a race war is a popular one, but the simple fact is that the ANC and its soldiers could not have defeated the SADF in a straight-up battle. South Africa gave up apartheid because they worked out deals that assured their survival in a nation with majority rule. Don't do that, apartheid doesn't end.
What makes the idea possible is the fact that South Africa's mixed-race population stood with the whites in the 1990s, in large part out of fear that Mandela would be another dictator. The combined population of the two groups in South Africa in the early 1990s is about 8 million, and they are a demographic majority in nearly all of the former Cape Colony, which outside of Cape Town and its immediate environs is fairly sparsely populated. If South Africa's black population and government goes out and tries to run the whites out in the 1990s, the first thing they will get is an immediate (and very, very nasty - we're talking six-figure body count here) response from the SADF to try and put the resistance down, and should that fail completely, a withdrawal out of the Transvaal, Free State and much of Natal, though the SADF would probably try to secure Johannesburg (for the infrastructure), Durban, Port Shepstone and Port Elizabeth (for the ports and terminals) and the Cape Colony areas where the people supporting them form an outright majority. Joburg and Pretoria are effectively indefensible, so I suspect that the SADF, probably backed up by a huge number of drafted volunteers, would hold out in Johannesburg just long enough to take everything they could out of it, then bail down the highway to Durban.
Durban is going to be the second place where a mess rules. Keeping it allows South Africa to control much of the exports of southern Africa, and for that reason, and the Indian population who lives there (not liked by the black African tribes much either), the SADF would want to keep Durban and Pietermaritzburg up the road. That would be very hard to do, but probably possible, if the SADF can secure the coastline in their favor.
Cape Town will see problems early, but after that not much. I'm not sure whether they'd kick out the black populations on the Cape Flats - part of me says that vengeance would say yes, but another part of me would think that as they would be a small minority and it would serve lots of good PR purposes not to boot them that they would be allowed to stay, though those kicked out of the rest of South Africa probably would not be too kind to them. Either way, a rump RSA remains, capital undoubtedly in Cape Town and with nothing but animosity for the groups that kicked them out of land perceived to be theirs. Over time, South Africa would probably see its black neighboring nations rather like Israel sees the other Arab states.