Mangawhai Harbour is a shallow tidal estuary that fills and empties twice a day through a narrow channel at the Heads. At high tide it is a calm sheet of water that reaches several kilometres inland; at low tide it becomes a maze of sandbars, mangrove islands and wading-bird habitat.
The harbour is the centre of the area's birdlife. Variable oystercatchers, white-faced herons and New Zealand dotterels all nest along the sand spits, and the dotterel colony at the harbour mouth is one of the most successful in the country. Kayaking on a rising tide is the best way to see it — launch from the boat ramp at the Heads and paddle up the channel. The current is strong near the entrance, so plan around the tide.