If you want to catch a big one Copeton dam near Inverell is a good starting point, but it could take many trips before you find a goodun.
If you just want to catch a cod try to get on to a patch of one of the border rivers (Severn etc) with a canoe or yak, and you will generally catch more fish, just mainly 50 to 70cm models with the occasional big girl.
bout now is always a good time