Also, how do you plan to travel from A to B when you are here (if you don't go the tour route).
Queensland and more so the Northern Territory are HOT during half/most of the year.
Queensland accomodation along the beaches/near attractions will be expensive.
Same with Sydney.
If we go on a holiday (as in, going to a town in Victoria), we stay in a caravan park but we use the cabins. So, as an example, we stayed 7 days in a town 2 years ago and we paid $500 to $600 for the mid size family cabin. That was hundreds cheaper than staying in a motel or hotel.
That example is country Victoria, so staying at a popular tourist destination will be much more expensive.