The mainland part of Auckland City occupied the Auckland isthmus, also known as the Tāmaki isthmus. The Waitematā Harbour, which opens to the Hauraki Gulf, separated North Shore City from the isthmus. The Manukau Harbour, which opens to the Tasman Sea, separated Manukau City from the isthmus. The distance between the two harbours is particularly narrow at each end of the isthmus. At the western end, the Whau River, an estuarial arm of the Waitematā Harbour, comes within two kilometres of the waters of the Manukau Harbour on the west coast and marks the beginning of the Northland Peninsula. A few kilometres to the southeast at Ōtāhuhu, the Tāmaki River, an arm of the Hauraki Gulf on the east coast, comes just 1200 metres from the Manukau's waters. Being part of the Auckland volcanic field, much of the isthmus is mantled with volcanic rocks and soils, and several prominent scoria cones dot the isthmus.
