We dove with Mike Ball out of Townsville. Did the Yongala twice, then headed straight out. The reefs were beautiful, teaming with life.
Do you have any concept of just how HUGE Australia is? Most towns are on the coast with large distances of nothing but fences and kangaroos between. they also drive on "the other" side of the road. The highway that goes around the coast is full of "roadtrains," linked together monster trucks that dominate the roads.
Best deal on airfare is to go through a travel agency and get a package. We spent 6 weeks. Our week in Sydney (hotel, breakfast, tours, museum entrance, etc) plus airfare was less than what we would have paid for airfare alone. You don't need a car in Sydney. Great local transportation, lots of tours. You don't want to drive to Ayres Rock by yourself. Take a tour, get there in a big comfy bus where someone else worries about driving in the desert, running out of gas, etc. The interior is lovely, but desolate and dangerous if you don't know what you are doing. Cairns is also awesome with lots of hiking and biking.
It's a wonderful place to visit!