Ahuriri Lagoon (Māori: Te Whanganui-a-Orotū) was a large tidal lagoon at Napier, on the east coast of New Zealand's North Island, that largely drained when the area was raised by the 1931 Hawke's Bay earthquake.
Before the earthquake, the lagoon stretched several kilometres from north to south, and covered roughly 4000 hectares (ha), or 40 km2.[1][unreliable source?] The Tutaekuri River flowed into the southern end, and the Esk River into the northern end. Following the earthquake, the Esk was no longer able to flow into the lagoon and ran more directly to the sea. The Tūtaekurī still flowed into the lagoon after the earthquake but it caused flooding for the next few years, and by the end of the 1930s it had been diverted away from the lagoon to enter the sea at the mouth of the Ngaruroro River.[2]
The land rise in the earthquake drained much of the lagoon, leaving a smaller estuary. Land reclamation and drainage work further reduced the estuary to its present size of 470 ha.[3][4]