Simply put, because they didn't. Often I find that people tend to equate 'Colonial Australia' with colonial Sydney.
Whilst there were incidents of conflict with Indigenous Australians, there were also a lot of incidents of cooperation. In most cases though, the settler just tended put push the original inhabitants further from their areas.
The problem with this question is that it doesn't take into account the cultural topography of colonial Australia. Australia wasn't a single nation state during colonisation but actually a number of separately administered colonies. Whilst Sydney may have used a lot of convict labour, this wasn't necessarily the case in Melbourne, Adelaide, or Perth which was if memory serves, almost entirely free settlers.
The Indigenous people didn't have fixed settlements but were nomadic hunter-gatherers who moved around the country according to the seasons. You couldn't just go capture yourselves some "Aborigines". It's not like North America or Africa where you had visible fixed settlements. If you've ever seen the terrain of the Australian Bush it's not something that lends itself to finding people, even those who want to be found.
In short, as mentioned slavery was illegal, it was too difficult to manage as an institutionalised practise and the settlers had enough trouble finding enough for themselves to eat, let alone support a meaningful population of slaves. This was not a problem the Indigenous Australians had.
You would find later that the Indigenous Australians did live in and work in slave-like conditions. But they were still paid, and often given (bare) subsistence. So it is a matter of semantics.
If you are interested in early Australian colonial life I highly recommend "The Colony"