You already know the answer… the indigenous people were not actually people in the eyes of the colonists.

I found hundreds of maps of the First Nations People of Australia, including language groups, but was not able to discover any official map, which is not surprising given how counting the population worked in Australia. All maps show that the "uninhabited" area was just as densely populated as the rest of the continent.

(Trigger Warning: heavily racist policy descriptions)

How the population of Australia was counted: https://www.nma.gov.au/defining-moments/resources/first-nations-peoples-census