The aborigines were living in Australia for 50,000 to 65,000 years, but within a century of the arrival of the Europeans (the British), they were mostly eradicated. Here’s an excerpt from Jared Diamond’s book Guns, Germs, & Steel (Chapter 15, “Yali’s People”):
“Australian Aborigines, of course, stood in the way of European food production, especially because what was potentially the most productive farmland and dairy country initially supported Australia's densest populations of Aboriginal hunter-gatherers. European settlement reduced the number of Aborigines by two means. One involved shooting them, an option that Europeans considered more acceptable in the 19th and late 18th centuries than when they entered the New Guinea highlands in the 1930s. The last large-scale massacre, of 31 Aborigines, occurred at Alice Springs in 1928. The other means involved European-introduced germs to which Aborigines had had no opportunity to acquire immunity or to evolve genetic resistance. Within a year of the first European settlers' arrival at Sydney, in 1788, corpses of Aborigines who had died in epidemics became a common sight. The principal recorded killers were smallpox, influenza, measles, typhoid, typhus, chicken pox, whooping cough, tuberculosis, and syphilis.”
When Captain Cook visited Australia between 1768 and 1779, it is estimated that the aborigine population was 750,000. Ninety percent were dead by 1920. The ships carrying British settlers reached Australia on January 26, 1788. Within a few years, the British army and police forces were using guns, germs, and other tactics to eradicate the aborigines. There is no dearth of controversial theories on how the germs (Eurasian diseases) had spread with such rapidity in the aborigine communities. A number of cases of deliberate poisoning of flour and other consumables given to the aborigine communities have been documented.
There was no political unity among the aborigines. They were splintered into thousands of small and mostly unarmed tribes. The smaller aborigine tribes were destroyed more casually by the groups of European armed private citizens. The irony is that several treaties were signed between the British and some of the larger aborigine communities, but the British kept breaking these treaties. They kept expanding into the continent, driving the aborigines out of their traditional areas. In the 1850s gold was discovered in Australia. Many small aborigine tribes were dispossessed and decimated when thousands of European immigrants, who wanted to become rich by finding gold, flooded the continent. Australia became a part of the Western Empire, and the aborigines became nonpersons in the land where their ancestors had lived for 65,000 years.