You would need a HUGE tank as they need a lot of room. Something else you need to know is that freshwater stingray's are much more venomous than saltwater stingrays as a rule so you must be very careful. The tank you would need to house a fully grown stingray would have to be custom made because typical tanks are nowhere near ideal for an adult. You would need sand as the substrate, you'd have to watch the parameters very closely. I also have no idea if it's even legal to keep one in Australia, and what the laws are there for that sort of thing.