Realize that we aren't all coming down on you for wanting to keep them, but just trying to warn you for your own sake and the sake of the hobby we all share. Recently there has been a lot of talk about banning all imported fish species and essentially killing the hobby of fishkeeping, all because of the issue of
Irresponsible fish keepers dumping their fish into their native waters where they become invasive. And Florida has seen some of the worst of it. If you get caught, not only will you probably get a hefty fine, but you will be adding fuel to the fire to ban keeping any non-native fish. Florida doesn't even allow public aquariums and zoos to display piranhas. They take this ban seriously.
I've wanted piranhas for years, too, but I live in Louisiana where they are also banned. Hopefully one day I will find myself living in a state that allows them, but until then I won't. They could destroy our ecosystem, too, and I am happier knowing that they aren't allowed so it is a lot harder for them to be acquired here. And yes, of course some people will have them where they shouldn't - there will always be a black market when something is banned.
I'm pretty sure we aren't aloud to keep them in Australia but you might be able to keep them wherever you are 
