I agree with Red, you just can't go past a live aboard trip out from Cairns or Townsville in northern Australia. The diving is supberb. The reefs off Townsville are a little better (my opinion)

than off Cairns. Also Townsville has the Yongala wreck which is a must do dive if you are in Australia. There is a giant cod on the Yongala nicknamed "VW" as it is the size of a bug.
The following is a trip report done by my partner a couple of months ago:-
Hi All,
Just came back from a purrrfect weekend, diving the Yongala Wreck on the GBR, what a magnificent dive site, the marine life is just incredible. On the second dive of the day we were gearing up for our next dive when 3 humpback whales surfaced next to the boat and hung around for about 5
minutes. The wreck is covered in beautiful soft corals you can hardly see any of the super structure and every marine creature from miles around has made the Yongala its home. Its alive with all shapes, sizes, and types of marine life. On one dive alone I saw turtles, morays, eagle rays,a huge spotted bull ray, two magnificent lion fish, and a mass of schooling fish, and to make the memory complete my partner and I were about to ascend when I looked out from the wreck and saw a 10 -12 foot Tiger shark cruise by I was absolutely stoked. I had heard that there was the odd Bull shark and Tiger shark that frequent the wreck from time to time, it was my lucky day. If you ever get the opportunity to dive the GBR it is a must do dive, I just thought I would like to share this experience with you.
John
My partner (John) and I dive regularly on the Yongala and it is always amazing.
Hope you can make a trip to OZ.
Jude