I looked at the Colesberg park in September and it is a major NO No. As already suggested go to to the Aventura resort at Gariep dam. You have 2 choices of site.
1. Luxury with own bathroom and kitchenette away from the dam and no trees or
2. Standard sites under the trees near the water, but a bit of a walk UP TO the ablutions.
3. The distance from JHB is 600 km.+- 8 hours.
4. To avoid road construction from Gariep to C.Berg. When you come out of Gariep to the T junction, turn left to Norvalpont (not right the freeway) go to the next tar road turn right to Norvalspont go over the narrow bridge over the Orange river,thru the vilaage to the T junction and turn right to C.Berg. This can save you up to 1.5 hours of stopping time. - SAFE DRIVING.