Diamond claims that New Guinea and Australia were the scene of the "first mass extermination of large animals." There is some scholarly debate about this, because no bones of large animals have been found in these lands with evidence that they were killed by people. But it is indisputable that they died out in large numbers around 40,000 years ago, and according to Diamond:
Personally, I can't fathom why Australia's giants should have survived innumerable droughts in their tens of millions of years of Australian history, and then chosen to drop dead almost simultaneously...precisely and just coincidentally when the first humans arrived.
Diamond is persuaded that megafauna in Australia and New Guinea followed a similar pattern as elsewhere. When they first came into contact with humans, animals had not yet learned to fear them, and so people could easily kill the animals. There is substantial evidence that this happened elsewhere (including, in some cases like the dodo, fairly recently) and Diamond believes that the commonsense conclusion is that it occurred in Australia and New Guinea as well.
Source: Diamond, Guns, Germs, and Steel, 43.