Not sure, green chromoid?
Looks like a mangrove snapper, Lutjanus argentimaculatus.
There are fresh water snappers in Australia. There are some here in Florida too where I live. Catch them all the time. They can tolerate freshwater and eventually move to salt.
The ones from Australia are pure freshwater. Wes brought them in prior.
I'm not sure but I'd guess by the 12inch mark be in brackish water.