If you got it on weed, it is probably a Rock Cale. If you got it on prawn, or another meaty bait, it is probably a Kelpfish. Most of the time.
"Rock Cocky has a rounded (cockatoo-shaped) head, while the Eastern Kelpfish has a more pointy head."
http://ncmg.org.au/Story31.html
Most importantly, they both taste like sh*#.