Pukaki Lagoon is one of the volcanoes in the Auckland Volcanic Field. It has a 600 m wide explosion crater, with a surrounding tuff ring. After eruption about 65,000 yrs ago the crater filled with freshwater and became a lake. It was breached by the sea as sea-level rise after the end of the Last Ice Age about 8,000 yrs ago and became a tidal lagoon. This was dammed and drained and used as a speedway from 1929 into the 1930s (Henning's Speedway). It is now farmland.