If you are planning on snorkelling make sure you head as far North as you can. Port Douglas is a great place to base yourself, fly to Cairns.Fantasea do great day trip to Low Isles.
If you are looking for a holiday off the beaten track, head for Western Australia. Fab snorkelling at Ningaloo Reef (without thousands of other people), you can fly Exmouth from Perth.Beautiful beaches around Perth city and Margeret River for Wine, Esperance for out of this world sunsets. We camped at a lot at National Parks...Cape Le Grand is amazing. We travelled along the East coast and Victoria too but found our time in W.A. fantastic and felt we got the real aussie experience.
Sydney is an interesting city 3 days max and then head out to Blue Mountains. Melbourne much groovier and friendlier and loads to do around...fairy penguins on Phillip Is., Great Ocean Drive, Ballarat, Grampians..... on and on.
Enjoy planning but don't overdo it...stick to one country..I'd actually suggest stick to one state!!!