I can tell you know that when I did environmental science at Uni, we did field work at the Cook's River and it is *nasty*.
We also used to trade horror stories about it - my favourites are that all the pollutants in the river caught fire in the 1970s and burned for several days as they couldn't be put out (this one's a bit apocryphal).
The other one was that, when excavating for the airport rail link, tests done several metres down into the sediment were still revealing levels of heavy metals (mercury in particular) way in excess of any safety regulation.
I always found it quite sad that this river was more or less used as a chemical dump for years with no concern for the fish or any kind of concern for the environment or future use of the River. The sewer overflows STILL run into it and straight into Botany Bay - a disgrace in my book, but I don't think the Government has any incentive to change it. Really, I'd be wary of fish in Botany Bay, too, but you have to draw the line somewhere. As long as you don't eat fish five days a week from there, you should be ok. We've all gotta die of something!
So I guess the moral of the Cook's is: catch, but always release.