Hi Pierre,
There are some great spots to dive around Oz. The Great Barrier Reef is an obvious choice. There is also an awesome coral reef in Western Australia, a little less known but totally amazing.
Spots like South West Rocks, Jervis Bay, Eden, Coffs Harbour are awesome. Some great diving to be done there.
If you don't mind cold water diving head down to Tassie, Bicheno and Eagle Hawk Neck are simply superb and in my mind offer the best diving in Oz. Kelp forests, sea caves, very colorful sponges, big drop offs... real nice stuff.
Melbourne has some nice diving too, lots of wrecks and some cool reefs if the vis is good. Lots of nice shore dives where you can see weedy sea dragons.
Lots to dive, lots to see. It all depends on where in Oz you are and what kind of diving you want to do. But there are LOTS of options.
Feb/March is a good time of the year, if you want to head down south, as the water temp will be the warmest.