I see no one's responded to your request. Sorry. I know there are people who've been to the GBR who subscribe to RC, but most focus is on the SPS forum site, not this one. Regardless, I can tell you from Aussie friends that Cod Hole and the Ribbon Reefs (#1-10) are worth investigating, but they are only reachable by liveaboard. The reefs are about 60 miles north of Cairns. You also might try reaching out to EDR42 who submitted the great GBR photographs (see first post in this forum). EDR42 is obviously an expert, having lived and studied the Reef. You might also ask if the dive sites near Townsville are better than the ones accessible through Cairns since they are closer to the Coral Triangle (you can also dive the S.S. Yongala from Townsville).
Have a great trip!
Scott
PS Depending upon how much time you have, Australia is also the transit country for the Solomons, Vanuatu, New Caledonia, and PNG. If you take Air Pacific to Australia, you can do an extended layover in Fiji.