Whakamahi Lagoon is a significant coastal wetland and tidal lagoon situated at the mouth of the Wairoa River as it flows into Hawke Bay on New Zealand's North Island. This brackish environment, where freshwater from the river mixes with the saltwater of the Pacific Ocean, creates a rich and dynamic ecosystem. The lagoon itself is formally designated as the Whakamahi Lagoon Government Purpose (Wildlife Management) Reserve, encompassing approximately 137 hectares and managed as a conservation area. Its role as the terminal point for the Wairoa River means it receives the full flow of the river's catchment, which includes Lake Waikaremoana and drains a vast area of over 3,660 square kilometers. The surrounding coastline west of the lagoon is characterized by steep, truncated ridge systems rising 100-200 meters high, composed of young mudstones and sandstones. This geography funnels the river's output into the lagoon before it meets the open bay, creating a mosaic of channels, mudflats, and estuarine habitats that are critically important for both wildlife and fisheries.