First of all, Piranhas are not native to Australia, they are more native to the Amazon river and Latin America. Furthermore, they have not been introduced into the Australian ecosystem either. The only places that they live other than that are in zoos or owned by fish enthusiasts. However, the question of this article is whether you can have a pet pirana in Australia or not. The whole of Australia have classified the piranha as a noxious fish, and therefore are illegal to buy or own.
There are some species that are allowed in certain areas. However, none of them are the true piranha species. Therefore, it is better to stay on the side of legality and not buy or own one at all. Even zoos have special permits that allow only licenced premises to own piranha. That is a similar rule to that of chinchillas, and that they could damage the ecosystem if allowed to escape into the wild.