I have not dove in Austrailia but the snorkeling is fantastic. We went out of Townsville and Cairns both were suburb. You will want to dive though. There is so much more to see and you can get a lot more up close and personal with the flora and fauna.
In fact while I was snorkeling in Cairns four divers past underneath me, that is when I made the decision to start diving, I was so jealous.
Enjoy!