Problems with reddies is that in a lake, especially they can reproduce at phenominal rates if there are no established large predators present . The food chain becomes insufficient to support the huge numbers of reddies so they become "stunted". I witnessed this at Malmsbury reservoir several years ago, huge schools of reddies , but all about 100/150mm long . You can get some pretty big "reddies" in amongst the timber in Lake Eildon
Similar problem with carp, but not good eating. They are generally bottom feeders , sucking up mud, sifting food out, then expelling the mud, which increases the turbidy of the water, decreasing the oxygen content and inhibiting light for plant growth, they also spawn out massive rates of eggs that can stick to water birds legs and be transported to unaffected areas.
The good side to all this is if the fisheries dept's in the relative states stock the rivers and dams with large fingerlings of trout, murray cod, Macqharie perch and ( yellow belly,callop, golden perch) the natives and the trout usually come out on top after a year or so
cheers Keith :Y: