I understand that the said fish can be found in freshwater rainbow, and I aren't going to jump down your throat either. I can understand how you could be confused due to them being found in streams, etc. Where the confusion arises is that often brackish water fish swim upstream to spawn before returning to salted water. The juveniles then grow in the freshwater before gradually swimming towards the ocean and into brackish water. Alot of lfs (down here at least) sell brackish water fish as freshwater fish without realising that they need salt as they grow older. I know that alot of barra are found in freshwater lakes and dams in the N.T (and therefore probably QLD as well) as well from when I spent some time there, but alot of the time the fish have been introduced there, or trapped due to dried up streams which once led to the ocean (often in the wet season the streams will again flow, letting the fish swim to the ocean once more).
As for the bullrout, they are pretty poisonous with sharp spines and would likely damage a barra's throat if swallowed, and possibly lead to death as well. I personally don't think it would be worth the money really. And as others have said, Barra can get very, very big. I have personally seen specimens around 2 meters long in the wild and a good half meter or so wide. You would need a swimming pool to house a fully grown one, or at least a tank that held enough water to fill a swimming pool, something that just isn't practical for the majority of us.
I hope this clears things up a little for you and doesn't come across as being agressive, it wasn't intended to be.
