Manukau Harbour, second largest on the west coast of the North Island, has an area of about 150 sq. miles contained within some 240 miles of shoreline. The harbour originated in events that commenced less than 10 million years ago, when sea invaded much of the North Island. A large bay formed in the Manukau – Port Waikato area, possibly the result of northward tilting of the land toward a fault passing east through the present harbour entrance. Although this drowned area may have been connected by narrow straits to the Pacific Ocean, its development was essentially that of a bay into which the ancestral Waikato River flowed, depositing sediments and slowly extending the coast northwards. At the same time the current sweeping northwards along the west coast of the island was depositing sand in the quieter waters as it passed the threshold of the bay. The resultant bar grew until it emerged as Awhitu Peninsula, which, because of the scour of the ebbing and flowing tides, has not connected with the resistant volcanic rocks of the Waitakere Ranges. About 3 million years ago lava flows, erupted from centres in the Pukekohe-Bombay area, diverted the Waikato River to the west and, although at times it almost certainly followed the course of the present Waiuku River, the Waikato has since mainly discharged into the Tasman Sea. The strong, dominantly westward winds have drifted sand dunes up to a present height of nearly a thousand feet along Awhitu Peninsula. Within the last half million years the Manukau has been effectively sealed off from the Pacific Ocean by volcanism around the Tamaki Isthmus, except for transient connections at times of high sea level. Puketutu (Week's Island) and the neighbouring hills along the eastern side of the harbour, many since destroyed by quarrying operations, were formed in this period. Even since the Waikato River was diverted to the Tasman Sea, many small rivers and streams have continued to deposit sediments within the harbour so that it has long been shallow, with many changing sand bars, extensive mudflats, and fringing mangrove swamps. At low water only the larger channels are navigable, but the main restriction to navigation is due to the bars at the narrow harbour entrance where the mean spring low-water depth is only 3½ fathoms. Here shifting sand bars, combined with the heavy swell and breakers caused by the prevailing westerly weather, make navigation hazardous.