Well basically there is a large tidal flow in the river, meaning that alot of the junk that ends up in there gets flushed out twice daily.
Sewage released into rivers legally has to be up to a certain standard, and apparently that standard is safe for fish that are bound to be consumed by humans.
Greater worry would be in the upper stretches where the flow is less. ya get fertiliser run off, dead things, general eutrophication (forgot how to spell), bugs and so forth.
Downstream I can image there would be some oil deposits and heavy metals, but I guess if it were a problem, there would be signs everywhere saying dont eat the catch. And any fish cleaning tables provided at river side parks such as comslie seem to suggest that fish are able to be taken and eaten.
People here eat stuff out of the river all the time, so no probs if you COOK IT WELL.
HAving said that, the river got a D- or something in the latest test of its health. I think that has more to do with turbidity levels though