It used to be much higher. In 1911 it was some 20% of total population.
The Cape Colony (as opposed to the country as a whole) in earlier times likely had a much higher white population. A table in The Shaping of South African Society 1652 - 1840 (2nd Edition), p524, gives a figure of 70 775 whites and a total of 79 450 for slaves, Khoikhoi, bastards and Free Blacks for 1840.