Taking feral fish out of natural waterways - the right thing to do?

[kezz_lives_here]

In my area (Cairns, Australia), I have discovered there are livebearer fish types that have been 'released' (by some kind-hearted but dumb-witted people) into some of the local creeks and rivers here. They are really pretty fish in a tank, but are there fines for keeping feral guppies/livebearers in tanks at home?
Taking feral fish species out of nature to put into man-made environments such as aquariums; is it the right thing to do, or should they be exterminated?