Te Awarua-o-Porirua Harbour, commonly known as Porirua Harbour, is the largest estuary in the lower North Island, located just 20 minutes north of Wellington. This natural harbour is situated within the main urban area of Porirua City and is formed by two primary arms: the Onepoto Arm to the south and the Pāuatahanui Arm extending to the north-east, each approximately three kilometres in length. The harbour entrance is narrow, only a few hundred metres wide near Plimmerton, opening into the Tasman Sea. The Pāuatahanui Arm features the largest remaining estuarine wetland in the lower North Island at its head, where the Pāuatahanui Stream enters. The harbour's geography was shaped by postglacial sea-level rise drowning westward-flowing rivers thousands of years ago. It is a dynamic, shallow-water environment influenced by tidal flows and freshwater inputs, with ongoing sedimentation being a notable characteristic.