By the late Eocene, New Zealand had eroded even more. The extensive swamp area was now flooded (this would eventually form the coal beds), and most of the land had been eroded to sea level. The fragments of land still jutting out of the water would have been a few chunks of gently rolling plains. It was right in the path, however, of the nutrient-rich circum-Antarctic oceanic currents and thus provided a perfect environment for the prolific development of plankton life, as well as all of those higher up the food chain.

By the Oligocene, the eroding had reached its peak. Two thirds of the present New Zealand was flooded over, leaving only a thin archipelago. As can be imagined, life on land was hard at this time. Animals may have coped by developing different species. Other species, trapped on islands, "developed separate geographical races". The soil was probably leached, making plant life strenuous. Due to arduous nature of life on New Zealand at the time, only the toughest plants and animals survived, clinging to the last remains of the land.