Answer:
The reason that vast regions of Australia are underpopulated is quite simple. Lack of fresh water. Even along the coast (areas in white) where there is fairly high rainfall, water is expensive. Only about 5% of Australia has land that has viable underground water. And that water is heavily controlled. Every bore is licensed and taxed and monitored. In the center most underground water is salty. And unsuitable for humans, animals or plants.
It is possible to process salt water to make fresh water but it is very energy expensive. Large amounts of electricity is required to make a small amount of fresh water. Australia has lots of solar energy but so far this is also expensive to convert to electricity. So really we can say that it is the high cost of fresh water that prevents people from living in 95% of Australia.
