While there are many articles discussing the Australian Aborigines' de facto legalized slavery, or essentially free labor, the fact seems to be that they were never de jure slaves, as Africans were.
Why didn't England press the Australian indigenous Aborigine population into actual slavery?
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"
Moral issues aside, it isn't that easy to just impress an indigenous people on a land you've just run into into slavery. They tend to stay out of your way. The British, Colonial, and US population never managed to do so to the Amerindians. They had to import Negro slaves from Africa for their labor shortfall.
What made the situation in Africa different is that tribes would collect the slaves and sell them to the Europeans. Similarly, Muslim slave buyers met with Viking slave sellers in Russia and the Atlantic to procure slaves. They didn't have to collect them in their own land at great trouble.
Also, in the case of Australia England had a better source for labor closer to hand - forced and unforced immigration from their own population. Convict labor replaced slave labor until natural increase and technology solved the labor issue.
Because it was illegal. Britain made slavery illegal in most of its dominions, including Australia, in 1833.
Putting aside the fact that Britain outlawed slavery in 1833, the conditions for "slavery" were not nearly as favorable in Australia as in the American south.
Slavery made sense in the American South because slave labor could be turned into cash crops such as cotton, and to a lesser extent, sugar, rice or indigo. These in turn depended on trade routes to markets in Europe. The distance from Europe to Australia is a multiple (maybe twice) that of Europe to North America, meaning that this cash crop trade, and hence slavery, would not be nearly as profitable.*
Also, the Australian aborigines would not have made as good slaves as Africans, if for no other reason that it would have been easier for them to escape to their homeland (the "rest" of Australia), whereas the Africans were separated from their homeland by the Atlantic Ocean. Instances of enslaved Africans trying to return to Africa on e.g. the Amistad, were so rare as to be the stuff of legend.
*And that's after the building of the Suez Canal. The multiple was more like three times, before that.
Probably because when widespread colonization of Australia started, (in the 1790s) chattel slavery was on its way out and about to be outlawed, the British Empire formally abolished slavery in 1833, which was not that long after the first colonies in Australia were established. Secondly, the British relied upon another form of unfree labour that in some ways might be considered akin to slavery, penal servitude. Given that Australia's first British settlements were prison labor camps and surrounding staff housing, there would have been no need for chattel slavery even if the British had retained it.
