It is hard to write an overview of prehistoric Oceania because so much of it is yet to be discovered. Archeologists have discovered that people came to Australia, New Guinea and neighbouring islands as far north as the Solomons over 30,000 years ago, but excavations are still occurring to discover more.
There has been some research on the megafauna of Australia during pre-historic times. Most of these large marsupials (think Skippy on steroids) and birds became extinct during the Pleistocene Era which was approximately 20,000 years ago. It has been suggested that the Aboriginals arrived in Australia 60,000 years ago, so they would have co-existed with the megafauna. Some of the species include the largest kangaroo ever and an echidna the size of a sheep. There was also a large goanna-like carnivore and large flightless birds called Mihirung birds. It is believed that the megafauna were wiped out by a combination of the drought conditions that occurred about 18,000 years ago and the Aboriginal use of fire & hunting.
When the Aboriginals arrived in Australia, the center was covered by a huge swamp called Lake Mungo. It would appear that there were two tribes living either side of it & no evidence has been found to suggest that they ever met. Over thousands of years the center of Australia eventually dried up to become the red center we know today.
The Aboriginals lived in harmony with the land and adapted to it's changes. There was very little tribal conflict and there customs and art reflect little changes over thousands of years. There has been much discussion and debate over where the Aboriginals originated from, however recent DNA samples have shown that Aboriginal Australians are most closely related to New Guineans. This, along with archaeological evidence, would suggest that prehistoric Australia & New Guinea were occupied initially by the same Paleolithic colonization event which would have taken place before the land bridge between them submerged.
Plant microfossils examined at the University of Auckland provide direct evidence of a prehistoric civilization cultivating and gathering a number of plants. New Zealand's first farmers grew a number of plants including Kumera (sweet potato) and Taro, both of which are still popular in modern Polynesian cooking.
Foss Leach claims that New Zealand was first discovered less than 1,000 years ago from Eastern Polynesia. By the time these people had reached New Zealand they were expert fishermen. Fish plays a big part of the pre-historic Maori's diet, this is evidenced by fish fossils found at excavations around New Zealand. It also fits in with a lot of Maori folklore. It is interesting to note that out of the 750 fish species present in New Zealand waters only 35 species are found in pre-historic catches.
Foss Leach's claim that New Zealand was first discovered less than 1,000 years ago has been debated. At the moment a study is being funded by the Royal Sociey of New Zealand Marsden Fund to put a date on human arrival in New Zealand. This study is still underway at time of writing. The method they are trying to use is by by radiocarbon dating of kiore (the pacific rat) bones. This rat spread with voyaging humans so could not have preceded humans. The results should be very interesting.
Archeologists have found evidence of prehistoric trading taking place in New Guinea. They believe that Kori was the center place of trade for New Guinea. Clay pots were found on Koil Island, which has no natural clay deposits, however the village of Kep has a long tradition of clay pot manufacture. It is believed that these were traded for galip nuts.
There also seems to be formal pre-historic worship occurring between both the Koil Islanders and the people on the mainland. However despite the trading and the sharing of religion it appears that they spoke different languages.
There is also the statues of Easter Island and the ancient city of Mu'a in Tonga which has been compared to Stonehenge. At this stage the overview is still being writing as more excavations and studies take place.
Sources: metmuseum.org, University of Victoria, ozzyfrank.com, pnas.org, Polynesian plant subsistence in pre-historic New Zealand by M Horrocks, Fishing in pre-European New Zealand by Foss Leach, arceozoo.org, The National newspaper,