De acordo com o site do Consulado da África do Sul nos EUA, Visto de trânsito - 3 dias ou menos você não precisa de visto de trânsito. A partir desse link:
Travelers transiting through the following international airports are exempted from/do not have to be in possession of transit visas, but will be subjected to biometric capturing directly at the airport:
OR Tambo International;
Cape Town International;
King Shaka;
Lanseria (Johannesburg)
Please note that the above-mentioned exemption does not apply to deportees. Please also note that foreigners who are utilizing land ports of entry to transit through the Republic of South Africa and who are subject to South Africa visa control should apply for and be issued with port of entry visas.
Então, se você estivesse usando uma porta de entrada de terra, você precisaria de um visto de trânsito