With an area of 70 square miles (180 km2),[1] it connects the city's main port and the Auckland waterfront to the Hauraki Gulf and the Pacific Ocean.

[2] The harbour is an arm of the Hauraki Gulf, extending west for eighteen kilometres from the end of the Rangitoto Channel.

The westernmost ends of the harbour extend past Whenuapai in the northwest, and to Te Atatū Peninsula in the west, as well as forming the estuarial arm known as the Whau River in the southwest.

North Shore suburbs located closest to the shoreline include Birkenhead, Northcote and Devonport (west to east).

To the east of the bridge's southern end lie the marinas of Westhaven and the suburbs of Freemans Bay and the Viaduct Basin.

There are other wharves and ports within the harbour, notable among them the Devonport Naval Base, and the accompanying Kauri Point Armament Depot at Birkenhead, and the Chelsea Sugar Refinery wharf, all capable of taking ships over 500 gross register tons (GRT).

Smaller wharves at Birkenhead, Beach Haven, Northcote, Devonport and West Harbour offer commuter ferry services to the Auckland CBD.

This valley provided the harbour with a second entrance when sea levels rose, until the Lake Pupuke volcano plugged this gap.

The harbour also proved a fertile area for encroaching development, with major land reclamation undertaken, especially along the Auckland waterfront, within a few decades of the city's European founding.

[11] Taking the idea of the several Māori portage paths over the isthmus one step further, the creation of a canal that would link the Waitematā and Manukau harbours was considered in the early 1900s.

[13][14] While the harbour has numerous beaches popular for swimming, the older-style "combined sewers" in several surrounding western suburbs dump contaminated wastewater overflows into the harbour on approximately 52 heavy-rain days a year, leading to regular health warnings at popular swimming beaches, until the outfalls have dispersed again.