In 1996 South Africa got a new constitution, the Constitution of the Republic of South Africa Act 106 of 1996.
(See: Development of Constitutions in South Africa)
Section 2 of the Constitution says that the Constitution is the supreme law of the land. All laws have to follow it and cannot go against it.
(See: The Relationship Between the Constitution and Other Laws)
Provincial governments can make their own constitutions, but these constitutions cannot go against the national Constitution.