There are plenty venues with good fish, but it takes a lot of effort to find them. And trust me, when you've been through the mill trying to get access and then trying to figure out how to get the fish etc. you will also be secretive about your spots.
Some of the well known (public knowledge) spots that hold good carp are Bellwater Park, Doordekraal, Sonnestraal, Rondevlei (pay to enter, fish off the island), Zandvlei (a little dodgy nowadays) and Magic Forest. Of those mentioned the majority are a little sensitive with regard to whether or not you are allowed to fish and some are not all that safe either.
There are "private" venues that hold carp up to low 40's that I know of, some claim 50's, but I can't vouch for that.
@bassy, you either didn't read my post or you don't believe me I guess....go and see for yourself. A 4kg fish in that dam would be a good catch by the current standard.