Pacific islanders (Polynesians,
Melanesians, Micronesians) before contact with Europeans were able seafarers that had discovered and settled virtually all Pacific islands that could sustain permanent settlements, including rather isolated ones such as Rapa Nui (Easter Island), New Zealand, and Hawaii. Furthermore, there were empires spanning numerous islands spread out across hundreds of kilometers, such as the Tu'i Tonga empire. This gives the impression of a sophisticated seafaring culture that would extensively explore and regularly visit all noteworthy lands found in and around the Pacific ocean.
However, this does not appear to be the case. Instead, some island cultures would - for lack of resources for shipbuilding or for other reasons - lose the ability to build oceangoing vessels. This is the case for both Rapa Nui (Easter Island) and the Moriori of Rekohu (Chatham Islands). It is conceivable that they preferred isolation over maintaining contact with other islands, though the hardships resulting from changing their way of life and their subsistence economy may make this unlikely. But why were they not rediscovered by other Polynesians?
It seems that some island populations even became extinct when natural resources were exhausted. An example are the Polynesian settlements on Pitcairn and Henderson Island, though this has been linked to civil war on Mangareva Island with which they were apparently in contact.
Furthermore, if they could explore and settle almost every single Pacific island, why would they have missed the surrounding continents, which presumably would have been quite a bit easier to find? There is limited evidence for interactions with South America on a very limited scale, but there was apparently no established trade network let alone migrations of political interactions.
Regarding exchange between Pacific Islands and Australia, there may simply be no record of this. Two HB questions (this one and this one) regarding this resulted in the assessment that: 1. there are no known prehistoric contacts between native Australians and Pacific islanders (specifically Maori). 2. Pacific Islanders (whether Polynesians of Melanesians or others) would have had no interest in settling Australia (different climate from what they were used to). 3. They would not have had superior weaponry over that of the Australian Aborigines.
To what extent were Pacific island cultures in contact with one-another? What was the nature of their seafaring?
- Was it a continuous and extensive trade network with political interactions like in the ancient Mediterranean?
- Did such a trade network exist only in the core areas, such as in the the Tu'i Tonga empire, with outlying islands being isolated?
- Were interactions rather local, to neighboring islands only, with a few rare examples of daring explorers visiting far-away lands?
- Did this change over time with periods of extensive trade and integration and periods of the breakdown of inter-island exchange?
- Was ocean-seafaring done by only a small group (a caste? a tribe?) of the wider population?
- Or was, while the capabilities existed, no seafaring conducted for the purpose of trade (and consequently no need for continuous exchange over longer distances)?
Edit (March 10 2018): As suggested by @Semaphore I now limited the scope of the question to Polynesians. I do not want to discourage interesting insights about Melanesians or Micronesians or others, but maybe this will make the question easier to answer.