New Zealand's landmass above water is just the tip of a much larger continent below sea level called Zealandia. Its visible shape has continued to change ever since it broke off from Gondwana more than 80 million years ago and drifted to its current position in the South Pacific.
Based on geological and biological evidence scientists agree that the deepest submergence occurred during the Oligocene, around 25 to 23 million years ago. In the 1960s, Sir Charles Fleming sketched maps that depicted Zealandia's changing shape and shrinkage. He recognised that wherever there was limestone (essentially an accumulation of marine organisms), there must have been ocean at some point. His drawings of the Oligocene period allowed for a considerable amount of remaining land in the north of the North Island and the south of the South Island.
Recently, geologist Hamish Campbell suggested that New Zealand may have drowned completely during the Oligocene, based on the wide-spread deposits of limestone across much of New Zealand's modern landscape. However, others disagree, pointing to a rich fauna of freshwater invertebrates and DNA sequences of plants such as kauri that would be impossible to explain if all land had disappeared. Dallas Mildenhall, a palaeontologist at GNS Science, hopes to provide answers through his research project that investigates plant DNA, ancient pollen and plant fossils.