At the peak of the last glaciation about 20,000 years ago, temperatures were about 5°C below the current mean. Ocean water was locked up in massive ice sheets, so sea levels were 120–130 metres lower than they are today. This extended the land area of New Zealand and the North and South islands were joined. By 12,000 years ago Cook Strait was flooded and the temperature had warmed sufficiently, so that the sea level was only about 70 metres below present levels. The two islands became separate.
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Source: GNS Science