Mangere Lagoon is a lagoon

Lagoon

A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...

 in the Manukau Harbour

Manukau Harbour

Manukau Harbour is the second largest natural harbour in New Zealand by area. It is located to the southwest of the Auckland isthmus, and is an arm of the Tasman Sea.-Geography:...

, New Zealand

New Zealand

New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

. It occupies a volcanic crater or maar

Maar

A maar is a broad, low-relief volcanic crater that is caused by a phreatomagmatic eruption, an explosion caused by groundwater coming into contact with hot lava or magma. A maar characteristically fills with water to form a relatively shallow crater lake. The name comes from the local Moselle...

 which is part of the Auckland volcanic field

Auckland Volcanic Field

The Auckland volcanic field is a monogenetic volcanic field in the North Island of New Zealand. Basaltic in nature, it underlies much of the metropolitan area of Auckland....

. Oval and about 600m long, it has a small restored scoria

Scoria

Scoria is a volcanic rock containing many holes or vesicles. It is most generally dark in color , and basaltic or andesitic in composition. Scoria is relatively low in mass as a result of its numerous macroscopic ellipsoidal vesicles, but in contrast to pumice, all scoria has a specific gravity...

 island remaining in the centre.

In the 1930s efforts were made to drain the swamp for conversion to pasture

Pasture

Pasture is land used for grazing. Pasture lands in the narrow sense are enclosed tracts of farmland, grazed by domesticated livestock, such as horses, cattle, sheep or swine. The vegetation of tended pasture, forage, consists mainly of grasses, with an interspersion of legumes and other forbs...

, while in the late 1950s, earthworks for sewage sludge

Sludge

Sludge refers to the residual, semi-solid material left from industrial wastewater, or sewage treatment processes. It can also refer to the settled suspension obtained from conventional drinking water treatment, and numerous other industrial processes...

ponds in the lagoon removed the scoria cone and the crater was divided into ponds. Public demand for a better sewage treatment system eventually led to a land-based facility, and in 2003, the restoration of the area to a tidal lagoon was completed, with the sludge removed. The scoria cone was rebuilt with an extra flat portion added on the west side as an artificial bird roost.