Sometimes the stupidity is so thick, it is hard to realize that it is real.
There is probably not a person on the planet that does not have ancestors that both owned slaves and were slaves.
Slavery was a common institution in nearly all societies for many thousands of years. when the Europeans came to the Americas, slavery was already long established. Slavery was common in Africa, in Asia, in Europe, and in North and South America. I do not recall it being mentioned in Australia, specifically, but women were virtual slaves there before the Europeans came.
It is hard to keep male slaves when you are a nomadic hunter-gatherer society. Enemies that are vanquished are simply killed.