Apart from public records, i.e. records generated by government, South African archives repositories also hold private collections presented to them for preservation. These accessions, as they’re called, include biographical and genealogical information, family trees and unpublished manuscripts of family histories which could save you time if someone else has made a study of the family you’re researching. Bear in mind that manuscript (or even printed) family trees shouldn't be taken at face value and it's advisable to check all the details presented.
To check, search the MAN database on NAAIRS for manuscripts, collections of photographs, maps and other material held by all South African repositories, as well as material available at South African museums, libraries and universities.
All these can be discovered at www.national.archives.gov.za