During the Pleistocene the alternate locking up of vast quantities of sea water to form ice caps, followed by its release in interglacial episodes, caused sea level to rise and fall hundreds of feet all over the world. In some parts of New Zealand where the rocks were soft the sea was able to cut broad shelves at times of high sea level. In glacial episodes the sea level fell, extending the coasts by exposing parts of the continental shelf. The last map of diagram 7 illustrates New Zealand's coastal outline at the peak of the Otiran glaciation. The sea then retreated furthest, exposing most of the continental shelf, temporarily increasing the land area of New Zealand and joining together the North and South Islands and Stewart Island.
During its world-wide rise since the Ice Age, the sea has drowned the mouths of many New Zealand river systems, forming the intricately embayed harbours of the northern North Island and of Otago and Banks Peninsula; the fiords of Fiordland were also flooded.