The answer to this can be found in Chapter 15.
First, we must think about why this question uses the word “conversely.” This word seems to be contrasting the Australian aborigines with some other group. To answer the question, we must know which group this is. As it happens, the group is the natives of New Guinea. These people were not effectively conquered by the Europeans while Australian natives were.
As is usual in this book, Diamond attributes this to geography. He says that New Guinea was not a hospitable place for Europeans, their plants, and their animals. By contrast, Australia was hospitable. Australia’s climate was much more suited to European people and agriculture. It had fewer tropical diseases. These factors made it much easier for European people to live in Australia and to transplant their agriculture to that land.
So, Australian aborigines lost out to the Europeans and New Guineans did not because Australia was better suited for European colonization and settlement.