Please be advised that an international drivers permit is now required in order to drive a rental car in South Africa.
Extract from the Automobile Association of South Africa's website:
Please be advised that the South African Traffic Department are now enforcing a law which was passed in 1998 that travelers to South Africa wishing to rent a motor vehicle whilst in this country must be in possession of a valid "International Drivers Permit/License". Should travelers neglect to obtain this document before leaving their country of residence, they will not be permitted to hire a vehicle in South Africa.
International Drivers Permits can be obtained from the Automobile Association of Zimbabwe. The person applying for the permit must go into AA with 2 passport sized photographs and their original Zimbabwean drivers license. They offer 2 options - an "instant" option, for USD 50 and a 7-day option for USD 30. You do not have to be a member of AA in order to obtain an international drivers permit from them.
Please refer any queries to Automobile Association of Zimbabwe - contact details are: