The chain consists of two large islands, and several smaller islands. Aorangaia and Archway Island lie to the southwest of Aorangi Island, and there is also a group of smaller rocky islets between the two main islands, the largest of which is Motu Kapiti. The Poor Knights Islands are the eroded remnants of a 4-million-year-old rhyoliticvolcano that is estimated to have been tall and in diameter.
Oceanography
Spring tide range for the islands is around, decreasing to a neap tide of around. The deep water around the island results in only moderate tidal currents. These are around the same magnitude as the prevailing shelf currents. In the general vicinity of the islands mean flows are around and run toward the southeast. A remarkable feature of the region is the large internal tides that occur. These are a form of internal wave driven by the local tidal flow forcing the stratification against sloping areas of the shelf face. The surface manifestation of these waves can be seen from space. These waves generate brief highly localised accelerations. Internal wave amplitudes of around have been observed, generating flow speeds as great as.
Environment
The islands are protected as a nature reserve and a permit is required to land or tie boats up. Permits are usually granted only for scientific research. A notable native plant of the islands is the spectacularly flowering Poor Knights lily, which has become a popular garden plant. Feral pigs, which had roamed Aorangi since the departure of Māori in the 1820s, were exterminated in 1936. The islands have been identified as an Important Bird Area, by BirdLife International because they are home to a breeding population of about 200,000 pairs of Buller's shearwaters. The islands contain rock arches and sea caves, including Rikoriko Cave, the largest cave inthe world by volume, with a cavern measuring and an opening large enough for small tour boats to enter. Rikoriko Cave measures, with a ceiling height of and extends deep below water. Tawhiti Rahi contains the Northern Arch, Middle Arch, and Maomao Arch, the latter being a popular diving location. Aorangaia Island's east–west rock arch resembles a long tunnel, while the aptly named Archway Island is bisected by two rock arches, with the larger Cathedral Arch about tall.
History
The islands were earlier inhabited by Māori of the Ngāti Wai tribe who grew crops and fished the surrounding sea. The tribe traded with other Maori. A chief of the tribe named Tatua led his warriors on a fighting expedition to the Hauraki Gulf with Ngā Puhi chief Hongi Hika in the early 1820s. While they were away, a slave escaped the islands and travelled to Hokianga where he told Waikato, a chief of the Hikutu tribe, that the islands had been left undefended. As Waikato had been offended by Tatua some years previous, he and his warriors set out on three large canoes to attack the islands. They arrived at the islands one night in December 1823 and soon overpowered the islanders in the absence of their warriors. Many islanders jumped off the high cliffs to avoid being taken as slaves. Tatua's wife and daughter were captured and taken to the mainland where a distant relative recognised the wife and helped the two to escape. Tatua returned to the islands to find a scene of destruction. Only nine or ten people were left on the islands, including his five-year-old son who had been hidden in a cave during the attack. The islands were declared tapu and Tatua left with the survivors and went to Rawhiti in the Bay of Islands where he unexpectedly found his wife and daughter.
