If all of Africa was stable, united, and had a common sense of identity, the continent could well be a superpower.
That is ASBs I'm afraid.
One way could perhaps be that Australia never becomes popular as a settlement destination for British settlers (for whatever reason, perhaps the first colonies fail, or France or somebody beats Britain to the punch). We'll see greater migration to South Africa.
There will still be apartheid, no doubt, but a white population that is either the majority, or forms a plularity of the population, we will probably see what happened in Aus and the American South in OTL. There will be some discrimination, but with numbers on the side of the whites, there the discrimination won't be as harsh or oppressive.
A greater British-descended population may also make a difference. For example, if one has to look at Rhodesia, although there was definitely racial discrimination, and blacks were treated badly, it could be argued that black Rhodesians were better off then black South Africans. The majority of white Rhodesians were descended from British settlers, while the majority of white South Africans are Afrikaners.
A bigger white pop in SA will also result in greater white migration, from Britain and the Continent (considering SAin OTL was a relatively popular destination anyway).
This ATL SA probably won't be a true superpower, but will definitely be a regional power, and perhaps a true Great Power, maybe on the level of OTL Brazil, India, or Germany.