If you have dual citizenship i.e. two passports, you have to use your SA passport to leave and enter the country.
SOUTH AFRICAN CITIZENSHIP AMENDMENT ACT, 2004 (ACT NO. 17 OF 2004)
The Amendment Act repealed section 9 of the South African Citizenship Act, 1995 as of 15th September 2004, which regulated the use of the citizenship or nationality of another country by a South African dual citizen.
This inter alia means that the system of exemption/letters of permission to make use of a foreign passport has now been terminated. No further applications in this regard may therefore be processed.
The repealed section 9 has been replaced with a section as specified in the new section 26B. Section 26B reads as follows:
“ A major citizen who-
1. enters the Republic or departs from the Republic making use of the passport of another country; or
2. while in the Republic, makes use of his or her citizenship or nationality of another country in order to gain an advantage or avoid a responsibility or duty,
3. is guilty of an offence and liable on conviction to a fine or to imprisonment for a period not exceeding 12 months”.