Ōkārito Lagoon is New Zealand's largest unmodified natural coastal wetland, covering approximately 3,240 hectares on the West Coast of the South Island. This expansive, shallow, tidal lagoon is a dynamic estuarine system fed by several waterways, including the Ōkārito River, which drains from Lake Mapourika and Lake Wahapo. The lagoon opens to the Tasman Sea, creating a rich mosaic of habitats from mature rimu forest and mānuka scrub to extensive mudflats and brackish channels. Its geography is shaped by seismic activity along the Alpine Fault, which can alter the estuary floor. The small settlement of Ōkārito sits at the lagoon's southern end, providing the primary access point to this vast, trackless, and ecologically significant water body.