Hi Buzzard
Since no one else has answered this, I'll give it a shot.
I think that it is a seaperch or cod (Family Serranidae). It doesn't look exactly like anything pictured in "Sea fishes of Southern Australia" being particularly deep-bodied, but probably most like a Breaksea Cod Epinephelides armatus- "Inhabits coastal reefs of south-western Australia, where it is commonly encountered by both anglers and divers.... colour extremely variable." record size 2.9 kg, 55cm, good eating.