Well, firstly, most of the slaves imported to North America and the Caribbean came from West Africa - a region which is much more populous and much more densely populated than Australia, and the same was true during the era when slavery was at its height.
Australia would have been a terrible place to attempt to gather slaves for several reasons. At the very top of the list is that there simply weren't that many people to enslave to begin with. Australia has never supported a very large population, and the people who lived there were nomadic and migratory, not sedentary, so it wouldn't have been as simple as just marching into a village and snatching everyone that could be found. The social system that existed in Australia was completely different from that of the one that existed in Africa at the time. Most slaves were prisoners who had been captured in wars with rival tribes or in raids conducted into rival territory, but many African chiefs were more than willing to sell their own people to Americans and Europeans for the right price. Because of the proximity of Europe to Africa, even Africans living deep in the interior knew of the existence of whites, and slavery among Africans themselves was not unknown, so the concept was understood by everyone. That wasn't the case in Australia.
There were Australian Aborigines living in the early 1900s who had never seen a white man before. It would have been quite impossible to successfully enslave a group of people that didn't have any idea what slavery was. They were not farmers, so there was no real work to be done. They didn't construct permanent dwellings or herd animals or do any of the things that require repetitive labour, so it would have made little sense for Australian Aborigines to enslave anyone. What would the slave be tasked with doing? If you don't have fields to sow and plow, if you don't have animals to feed and graze, if you don't have buildings and tools that need upkeeping, then what good would a slave do you? And because Australian Aborigines inhabited a tremendously large area and were spread over the extent of an entire continent, the various tribes and bands had little in common - linguistically they were very isolated. Any two Australian Aboriginal languages would likely have had about as much in common as do Swedish and Swahili, and neither could understand European languages, so communication would have been a nightmare.
And how do you enslave people that exist in some of the harshest environments known to man? Europeans had been in Australia for decades before the first successful crossing north to south was made. And the first successful east to west crossing came much later. Black Africans were much easier to enslave because there were vast numbers of them living along the coast where Americans and Europeans could bring their ships right in to the shore or take them upriver, but Australia is not blessed with many deep water ports and there are few navigable rivers. Most of the rivers that exist are seasonal. And because Australian Aborigines didn't live in permanent settlements, they would have been difficult to locate, nearly impossible to track, and wholly impossible to capture. Would Americans and Europeans have been willing to scour the deserts and arid plains and dense tropical forests looking for Australian Aborigines to capture? They'd been warring with one another since time immemorial so they knew how to defend themselves. They could find food and water in the most unlikely places and managed to survive under the unrelenting meridian sun in a land filled with dangerous creatures. There's no way that entire armies of Americans or Europeans would have been successful at capturing Australian Aborigines in great numbers if they had tried. And many whites would have been dead from the extreme temperatures, the venomous snakes and man-eating crocodiles and the lack of food and water.
And even if whites had somehow managed to enslave a few, most would have died in captivity. I can't imagine that any would have made the journey back to North America and the Caribbean in the dark, airless holds of the slaveships, crammed in like sardines. They would never have survived the voyages. Not to mention how incredibly expensive an endeavour it would have been. Have a butcher's at a map. Find Australia. Now, trace a line from let's say Australia's east coast (always the most populous portion of the continent), to the places in North America and the Caribbean where slaves were in the highest demand. Do you know how long it would have taken for a slaveship to sail from Sydney to Charleston? From Sydney to Kingston? That's a long and expensive voyage. You can't take people that have lived their entire lives in open air living off the fat of the land and shove them into a rat-infested rickety slaveship and sail them thousands of miles around the globe and feed them swill and fetid water and expect them to be prized specimens ready for the auction block when they arrive.
Americans tried enslaving Native Americans and it didn't work either. They were too proud to accept the life of a slave, and they were on home soil, so escape attempts could be successful. A Native American enslaved in New England could feasibly escape from a plantation in Georgia and make it home. But a Black African bought in Africa and put to work in Mississippi would not be able to get home. Australian Aborigines would never have accepted being enslaved. They would have refused to work, lay down and died. Their masters could have beaten them, tortured them, starved them, did whatever they thought might force them to obey, but it would have all been for nothing, because to a man, they would have dropped dead before they'd pick cotton or clean house or any of those things.
Whites never would have been able to enslave Australian Aborigines, no matter what they might have tried.