The harbour mouth is between the northern head ("Burnett Head" / "Ohaka Head") located at the southern end of the Waitākere Ranges and South Head at the end of the Āwhitu Peninsula reaching up from close to the mouth of the Waikato River. The mouth is only 1800 metres wide, but after a nine kilometre channel it opens up into a roughly square basin 20 kilometres in width. The harbour has a water surface area of 394 square kilometres. There is a tidal variation of up to 4 metres, a very substantial change, especially since the harbour, being silted up with almost 10 million years of sedimentation, is rather shallow itself.