Dale, I live in Sydney’s eastern suburbs (Bronte) and I would personally have some dives in Sydney with Dive Bondi - but then for your 4 to 5 days vacation I would fly to Gladstone and then get a boat to Heron Island. Its the only place you can stay actually on the reef, which means you can fit in three great dives each day if you want to - then have a cocktail on the beach at sunset and watch the fins of the little white and black tip sharks as the clean up the shallows.
Enkoy, Martin.