You can find the answer to this by reading Chapter 18. The best concise explanation for this is found at the end of the chapter, on pages 319 and 320 in the paperback edition of the book. There, Diamond tells us that Australian natives were weak compared to Europeans because they lacked the resources that were needed to create an agricultural society.
When Europeans came to Australia, they made it into a densely populated area based on farming. But they had to bring crops and animals from Europe and elsewhere to do it. Australia itself had some land that could be farmed intensively, but it lacked the domesticable plants and animals needed for such food production. This proves, in Diamond's view, that environment (and not people and culture) caused Australian aborigines to be weak compared to Europeans.
You can also answer this by reading in Chapter 15. A decent summary of his idea can be found on the last page of the chapter. His argument is as follows:
If people were the cause of these differences, we should see Europeans succeeding in creating a complex agricultural society where Aborigines failed. Many people think this is what happened, but they are wrong. Europeans did not succeed any more than Aborigines did in domesticatingAustralian plants and animals. They had to import their own plants and animals (and even then, the penal colony was on the brink of starvation for a long time). This shows that Europeans were simply lucky. Their societies arose in a place with many domesticable plants and animals. Aborigines were not lucky as they lived in a place without such plants and animals and therefore could not create complex agricultural societies that could become powerful.