A combination of events in Australian history were disastrous for the Indigenous population as the settlements spread;

• There were only tiny settlements to begin with;
• The settlements were so isolated from each other that communication with New South Wales or Britain was practically impossible;
• Most of the country was a land of 'anything goes' - Native Forces under the charge of White Leaders went into some - though not all - areas with the express intent of murdering as many natives as possible - with or without an excuse. 
  Few settlers not at the frontier were aware of these semi-secret operations. It was widely believed that any 'retaliatory' killing of renegade Aboriginals was justified. In other areas, the absence of any practical police force to help people follow the law, settlers became a law unto themselves.
• Most settlers were huddled around the first settlements or the capitals later established. As the cattle kings made their push thousands of kilometres into the interior, they moved further and further from the influence of the law.

The order in which each state was settled

A few comments and illustrations might help readers get a sense of the distance between communities, the size of the settlements, and the communication problems.

Australia is slightly smaller in area than the lower 48 states of the U.S.A. [i.e. mainland America without Alaska and Hawaii]

Although the two countries are similar in size, there is an ENORMOUS population difference. Here's a comparison 220 years after Australia was settled:

To put it another way, the population of Australia today is about 10% bigger than the population of New York State:

Australia was more or less cut off from the rest of the world until the Overland Telegraph was completed in 1872.

Approximate Route of Overland Telegraph 1872

The first railway trip across the Nullarbor - from East to West along the southern edge of the continent – was not until 1917: Camels were used to carry supplies ahead of each stretch of rail as the line was built. 

Drover

If anyone wanted to travel between the eastern states and Western Australia, the standard form of transport was ship.

This part of the highway was not fully sealed until 1976

The first car trip across the Nullarbor was reportedly made in 1912, and it was 1941 before there was a graded (unsealed road) between the West and the rest of Australia. The last section of dirt road was finally sealed in 1976; a full 75 years after federation.