Niau's lagoon is swampy, hypersaline and entirely enclosed. The narrow strip of land surrounding the lagoon is covered by marshvegetation. The enclosed lagoon area is 33 km2, making the land size at 20 km2.
The lagoon has an unusual green color.
The only human settlement on Niau is Tupana, population 226 (as of 2012[update]).
Niau is one of the few locations where the original Tuamotu tropical moist forests ecosystem[2] (known locally as feo) has been preserved. This mixed broadleaf forest[3] ecosystem has disappeared on almost all other atolls. The forest has been preserved in the central parts of the island. It contains several endemic species of plants and animals.
The small Niau kingfisherTodiramphus gertrudae lives now exclusively in the island of Niau. This critically endangered bird disappeared from the Gambier Islands long ago.