I would imagine that Israel might have helped RSA in its quest for a deterrent. Other African countries had tried to develop it, e.g. Nigeria and also the borders around it weren't safe for the country as the neighbors have provided safe heaven for the ANC and SA Communist Party to launch a war against the government. It was a Cold War and US also had aided SA in its war in Angola. CIA had provided intelligence and US government had given them weapons. I know that for sure, as I personally know some former officers of the SA Army at the time. US had supported unsavory regimes like Marcos in the Philippines, Mobutu in Zaire, Pinochet in Chile and most of military regimes in Latin America from Somoza to Stressner to the Duvaliers. All in the name of the cold war. New Zealand and UK for years had sports contacts like in rugby or cricket. On the other hand Israel needed uranium and place to test its weapons. It needed a deterrent against its many enemies. And SA wanted something in return. It has nothing to do with Israeli government or its people support of apartheid regime. I personally saw the demonstrations of Israelis against SA governments policies new SA Embassy at Kaplan Street in Tel Aviv. It's just a case of Realpolitik.
Law expanding definition of terrorist activity passes first Knesset reading (Haaretz)
from the article: Israel denies offering nuclear weapons to Apartheid South Africa