Entry regulations for South Africa changed on 1 July 2005. By entering South Africa you will be issued a tourist visa although you are doing an internship. So usually you do not need an official visa issued by the embassy. For getting the visa an important fact is, that you don`t get paid for your internship. If you got a salary you would need a working permit. However, getting a working permit is fairly difficult and in general out of question for interns.
Generally it works like this:
At passport control at the airport they issue a tourist visa with the notation of an internship. Holding this you are allowed to stay in South Africa for 90 days. If you are doing an internship for six months, you can extend the visa at the "Department of Home Affairs" once for another 90 days. Due to these regulations your stay in South Africa is strictly limited to 180 days.