Provided your home country licence includes your photo and signature and is in one of S.A.'s official languages, there is no legal requirement for an international drivers license.
The official SA language list includes English, so that covers US, Australian and UK licenses and many others. Even non-English licences are OK, provided there is an attached translation by a "competent authority" (whatever that may be).
Of course, if you are pre-booking your car rental, it’s worth asking your specific car hire company if they have different requirements from the norm – that avoids hassle and stress when you get to them! Sometimes car rental companies seem to be a law unto themselves.
Bear in mind also that most car hire companies here require that you be at least 23 years of age to hire a car.
And you need to be a credit card holder.
Some companies may ask whether your license has any endorsements.
What does that mean? Well, for example, in the UK or Australia you get points for driving offences such as speeding and after a certain number of points are reached, your license is revoked for a period of time. So, do you have any ‘points’ on your license?
Another endorsement would be if your license stipulates that you need to wear spectacles or contact lenses while driving.