The answer to this can be found in Chapter 17. Basically, Austronesians diffused into the Pacific Ocean and came to dominate in those areas where the natives had not already developed agriculture. Diamond argues that it was environmental factors that determined whether natives of a given area developed agriculture. The environment, then, determined which areas of the Pacific would come to be dominated by the Austronesians as they diffused into the Pacific.
Diamond argues that some areas of the Pacific were less conducive to the independent development of agriculture than others. Indonesia, for example, was a poor area for agriculture and so its natives had no agriculture and did not even have polished stone tools. Because of this, they were easily dominated by the Austronesians as they diffused.
So, Austronesians diffused through the Pacific and settled in every place where agriculture had not previously been feasible for environmental reasons.