Paul,
Do you want tropical or temperate water? Do you want easy vacation diving?
QLD:
Obviously there's the GBR, dive from Cairns, don't bother with the Whitsundays. If you have the time and the budget, try and dive the northern Ribbon Reefs and get out to Osprey Reef in the Coral Sea. Well worth every penny if you can stretch for it.
Just south of Townsville is a small town called Ayr - dive the Yongala - definitely a Top 10 wreck dive in the world. Just amazing.
Sunshine coast has the ex HMAS Brisbane.
NSW - fish rock cave.
VIC - Great diving from Portsea / Queenscliff. Nice easy - moderate dive sites inside the bay, moderate to challenging sites outside the heads (weather permitting). The J class submarines are great if conditions allow. Also has the ex HMAS Canberra.
TAS:
Some amazing diving including kelp forests, caves, underwater granite cliffs, the Nord wreck are all accessible from the Tasman peninsula (near Port Arthur).
SA:
Ex HMAS Hobart (watch out for certain big fish in the area though).
Cage diving (if you DO want to see said big fish).
Cave diving around Mt Gambier if suitably qualified (not to be taken lightly, as you will not be allowed to dive without appropriate qualifications, and you'll probably die if you ignore safety rules).
WA:
Ningaloo Reef. Although many find the whale shark spotting to be over the top.
I won't post commercial content - but feel free to PM if you want some suggestions on who to contact for further info in various areas.