The island is 24.9 hectares (62 acres) in area, and lies 3 kilometres (1.9 mi) south of the suburb of Petone and the mouth of the Hutt River.

Ships arriving in Wellington Harbour with infectious passengers or crew would disembark them at Matiu / Somes Island for care and treatment before berthing in the city.

Legend has it that Matiu and the nearby Mākaro island received their original Māori names from Kupe, the semi-legendary first navigator to reach New Zealand and return home with knowledge of the new land.

Since then the board has adopted the formatting convention of placing a space before and after the slash, so the official name is now written as "Matiu / Somes Island".

[8][9] The island is 24.9 hectares (62 acres) in area,[10] and lies 3 kilometres (1.9 mi) south of the suburb of Petone and the mouth of the Hutt River.

[13] A distinct gully runs from the south of the former quarantine station and terminates at the sea on the southern end of the island between two largely forested ridges on either side to the east and west.

[16]: 58–59 [17]: 22–23  The water level in the harbour was much lower 20,000 years ago, and the ancient Hutt River used to flow down a paleochannel to the east of the Matiu / Somes Island ridge as far as present-day Kilbirnie.

[25][26] Matiu / Somes Island and the surrounding landscape of Wellington is dominated by grey sandstone and darker mudstone sequences, together commonly known as greywacke.

[32] Tube fossils have also been found at other nearby locations such as Princess Bay and Sinclair Head, along the shore platform of Wellington's south coast.

[1] In November 1835 Ngāti Mutunga people, affiliated to Te Āti Awa, seized the ship Lord Rodney at Wellington.

The crew were tied up and the Lord Rodney's captain John Harewood was forced or bribed to take a group of hundreds of Māori to the Chatham Islands.

[38] At various times throughout the 19th and 20th centuries the island hosted quarantine facilities for both human immigrants and animals, and enemy alien internees during wartime.

[42] During the influenza pandemic of 1918–1920, a few people were quarantined on Matiu / Somes Island, but there was general agreement that the facilities were completely inadequate.

[59] In March 1915 two prisoners escaped from the island by swimming to Petone,[60] where they turned themselves in at a police station, seeking to alert the authorities to allegedly poor treatment of internees.

[58][63] Although in general the inquiry report found an absence of evidence to support charges of ill-treatment, it did make some recommendations to improve conditions for internees, and noted the use of "disrespectful language" by the camp overseer.

[64] On 29 August 1939 Matiu / Somes Island was handed over from the Health Department to the Army and again shifted from quarantine station to internment camp, with the first group of internees arriving in late December 1939.

Three men escaped in November 1941 in a boat stolen from the island's caretaker and made it to the Akatarawa hills before hunger forced them out to buy food and they were rearrested.

[69] The Government moved the internees to a camp at Pahiatua, but in September 1944 this was needed for Polish refugee children so the prisoners were sent back to Matiu / Somes Island (apart from the Italians who had been allowed to return to their families after Italy signed an armistice in March 1944).

The transfer of ownership was part of cultural redress for Taranaki Whānui included in the settlement of their claims against the Crown for breaches of the Treaty of Waitangi.

[77] A small team of rangers lives on the island to maintain facilities, manage visitors and volunteers, and monitor bait stations.

The effects of grazing over a period of 125 years meant that by 1976 the island was mostly bare, with the exception of some exotic trees, including macrocarpa that had been planted for shelter.

[84]: 12–19 At about the same time, members of the Lower Hutt branch of the Royal Forest and Bird Protection Society became interested in revegetation of the island.

[84]: 74 Species introduced or reintroduced to Matiu / Somes Island include Cook's scurvy grass, northern rātā, large-leaved milk tree, taupata, hebes, ngaio, and five finger.

[94] However, the robins did not thrive, possibly because the regenerating forest does not yet provide sufficient damp leaf litter year-round to sustain an adequate population of invertebrates as food.

[95] However, in the 2020/21 season, there was high mortality of penguin chicks, attributed to starvation caused by warmer sea temperatures making it more difficult for adults to find and catch fish.

A solar-powered speaker system was installed to transmit fluttering shearwater calls each night, and has attracted wild birds to Matiu / Somes Island.

[116] Matiu / Somes Island is an increasingly popular tourist attraction and educational resource for local schools, with about 15,000 visitors per year.

[121] Award-winning New Zealand novelist Maurice Gee's novel Live Bodies was set in part on Matiu / Somes Island, with the main character spending time interned there during the Second World War.

In 1998, he co-wrote and performed in a play, Eulogy, that was based on the story of Samoan and German prisoners interned on the island during the Second World War.

[124] New Zealand author of fiction and non-fiction David McGill's spy novel The Death Ray Debacle is based on a true story about Victor Penny, an Auckland bus garage attendant and amateur radio enthusiast who in 1935 managed to convince government authorities that he could produce a 'death ray' that was capable of stopping an army, immobilising trucks, and bringing down enemy aeroplanes in flight.