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Lake Ellesmere is located in the Canterbury region of the South Island of New Zealand. It is a broad, shallow lake located directly to the west of Banks Peninsula, separated from the Pacific Ocean by a long narrow strip of land called Kaitorete Spit. It lies partially in extreme southeastern Selwyn District and partially in the southwestern extension of the former Banks Peninsula District, which now (since 2006) is a ward in the city of Christchurch.

The lake covers an area of 180 km² (70 mi²), and is the largest lake in the alluvial fan of the Canterbury Plains. The main river that feeds the lake is the Selwyn. The ecosystem of Lake Ellesmere is severely threatened as a result of human activity on the Canterbury plains.

The rarely used Māori name is Te Waihora, meaning spreading waters (compare Lake Waihola). The lake bed was returned to Ngāi Tahu ownership under the Ngāi Tahu Claims Settlement Act 1998.

The lake has suffered from eutrophication since the 1970s and a 2010 report on lake water quality deemed it to be the second most polluted lake in New Zealand based on the trophic level index.