From somebody that has done over a thousand dives along the east coast of Australia I would say anywhere you go in New South Wales and Queensland you will have great diving, it all depends what you want to see.
Sydney has world class diving where you can see weedy sea dragons, blue gropers, grey nurse sharks, port jackson sharks, wobbegong sharks, giant cuttlefish, sea horses, and the list could go on and on. South of Sydney there is great diving at Bass Point, Jervis Bay, Ulladulla, Batemans Bay, Montaque Island, Merimbula, Eden and many towns inbetween. From Sydney north to Brisbane there is also wonderful diving at Port Stephens, Seal Rocks, Forster, South West Rocks, Coffs Harbour, Byron Bay and Tweed Heads.
Of course Queensland has great diving on the Great Barrier Reef and many areas south of the reef, like Mooloolaba, Rainbow Beach, Bargara and even Brisbane - it all depends what you want to see and how much time you have. If you want to see unique critters that you will not see anywhere else in the world, then spend more time in Sydney. If you want to dive on coral and see tropical species then head to the Great Barrier Reef.
I would suggest you surf the net for the destinations I have listed or buy a guide book.
Anywhere you go you will have a great trip.
All the best
Nigel