Cool, thanks for the info. I'll definitely do more research. We are only in Sydney for about 4 days, so if we do not get around to it, we will definitely be back there and to other parts of Australia again relatively soon I'm sure.
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One other thought on the GBR: we did a relatively "inexpensive" liveaboard out of Cairns last July that stayed on the inner reef--frequently within day-trip range. The sites were nice enough, but crowded, and loaded with juvenile fish. If we had it to do over again (and the budget, obviously), we'd go to the outer reef instead in search of the bigger stuff. On a 5-7k budget for the month, this might not be feasible, but worth noting. Additionally, I believe GBR boats also leave out of Port Douglas.