Chatham Island is by far the largest island of the Chatham Islands group, in the south Pacific Ocean off the eastern coast of New Zealand, — not exactly "halfway between the equator and the pole, and right on the International Date Line" but close enough for tourism marketing purposes. Chatham Island was named after the survey ship HMS Chatham which was the first European ship to locate the island in 1791 . It covers an area of 900 km².
The geography of the roughly T-shaped island is dominated by three features: two bays and a lagoon.
On the east coast is the even larger Hanson Bay, which stretches for the entire 35 kilometre length of the island.
The ten kilometre width of the island, between these two bays, is entirely illusory. Much of it is taken up by the large Te Whanga Lagoon, which drains to the sea in the southern half of Hanson Bay. This lagoon covers about 180 km², and drains several small rivers that rise in the hills at the island's south end.