New Zealand's geography: islands built on a moving plate boundary

Where the islands sit on two plates

The North and South Islands are the high ground of Zealandia, a mostly submerged continent that Te Ara dates as breaking away from Gondwana 85 million years ago. It sank slowly as it drifted into the Pacific and was then pushed up again by movement in the crust. About 25 million years ago the plates began to wrench it apart: cracks inside the continent joined into the Alpine Fault, and the land came to lie across two plates that slide past and into each other.

The direction of collision flips along the country. Off the east coast of the North Island the Pacific Plate is forced under the Australian Plate, and the subduction feeds the volcanoes of the central North Island. Beneath the South Island the Australian Plate is forced under the Pacific Plate, and the movement is taken up partly by uplift and partly by sideways sliding along the Alpine Fault, a line nearly 800 kilometres long that runs under many West Coast towns. It last ruptured in 1717 along 400 kilometres of its length, and its estimated recurrence interval is about 330 years. The transition between the two styles of collision runs through the top of the South Island and Wellington, the capital, sits in the middle of it.

Not all of the land is rising. The north-east corner of the South Island is subsiding, and the sea has flooded its old river valleys to make the Marlborough Sounds.

How far the country reaches beyond two islands

The two main islands and Stewart Island stretch about 1,600 kilometres across latitudes 35° to 47° south, and the country is nowhere wider than about 400 kilometres. The North Island and the South Island are 23 kilometres apart at the narrowest point of Cook Strait; Stewart Island lies 30 kilometres off the southern tip across Foveaux Strait. Counts of the islands run from about 600 to about 700, depending on the account, and together they give 15,134 kilometres of coastline.

The territory extends far past the main group. The northernmost point is Nugent Island in the Kermadecs, near 29° south; the southernmost is Jacquemart Island in the Campbell Island group, near 52° south. The Chatham Islands lie some 800 kilometres east of the South Island and keep their own time zone. The exclusive economic zone covers 4,083,744 square kilometres, more than 15 times the land area. The Australian mainland is about 2,000 kilometres west across the Tasman Sea, and the nearest foreign land to the main islands is Norfolk Island, an Australian territory about 750 kilometres to the north-west.

How fast the Southern Alps are rising

The Southern Alps run about 500 kilometres down the South Island, and 18 of their peaks pass 3,000 metres. The land east of the Alpine Fault is rising at an average of 1 to 2 metres a century, and Te Ara notes that erosion has kept pace, so the range has rarely stood much higher than it does now. In a large earthquake the rise comes as a lurch of several metres.

Aoraki / Mount Cook has been listed at 3,724 metres since 2014. It stood at 3,764 metres before a rockslide in December 1991 took the top off it, and Britannica, last updated in October 2026, still prints a figure of 3,754 metres. Mount Tasman is listed at 3,497 metres.

Ice did the rest of the shaping. Britannica counts more than 360 glaciers in the Southern Alps; the Tasman Glacier runs 29 kilometres down the eastern slopes of Aoraki, and the Fox and Franz Josef glaciers descend the western side. Every ice-carved landform in the range dates from advances and retreats in the last 250,000 years. The Ōtira Glaciation peaked around 18,000 years ago, and the hollows behind its moraines filled to make lakes Te Anau, Wakatipu, Tekapo and Pūkaki. In the south-west the sea flooded glacier-cut valleys to make Fiordland.

What the volcanic plateau holds

The North Island is lower and gentler, with narrow ranges in a north-east belt reaching about 1,700 metres. Its centre is the Volcanic Plateau, and its largest volcanoes are not cones but calderas, basins left by repeated eruptions over the last 1.6 million years at Taupō, Rotorua and Okataina. Lake Taupō fills one. The Oruanui eruption about 26,500 years ago ejected 1,170 cubic kilometres of material by one count; the geology account on Wikipedia gives 530 cubic kilometres of magma. The eruption Te Ara dates to about 1,800 years ago, and Wikipedia to around 180 CE, burned more than a sixth of the North Island.

Auckland stands on an isthmus between a deep harbour to the east and a shallow one to the west, and it is built on a volcanic field of dozens of cones. The oldest, Maungataketake, is about 50,000 years old, and the youngest, Rangitoto Island, rose about 600 years ago. Limestone country produces a different landscape again: the karst of the Waitomo District, where glowworms light the caves, and of Takaka Hill in Tasman. Most of the geothermal fields, including Rotorua and Waimangu, lie inside the calderas, and heat from the same zone runs several geothermal power stations.

Why the west is wet and the east dry

The mountain chains stand across the prevailing westerly winds. NIWA, the national climate institute, describes the West Coast of the South Island as the wettest part of the country and the land east of the mountains, just over 100 kilometres away, as the driest. Fiordland receives 5,000 to 8,000 millimetres a year, and up to 15,000 millimetres in isolated valleys. Central Otago and parts of the Canterbury Plains receive about 350.

The climate runs from warm subtropical in the far north to cool temperate in the far south, with alpine conditions on the ranges. NIWA gives mean annual temperatures from 10°C in the south to 16°C in the north, with July usually the coldest month and January or February the warmest. The gap between summer and winter is small near the coast and reaches 14°C inland and east of the ranges. Most of the country gets at least 2,000 hours of sunshine a year, and the midday summer UV index is often very high, reaching extreme in the north and in the mountains. Snow falls mostly on the mountains. It rarely reaches the coasts of the North Island or the west of the South Island, though the east and south of the South Island may see some in winter, and frost can form anywhere on a clear, still night. Temperatures fall by about 0.7°C for every 100 metres of altitude.

The rain that falls on the western slopes is what generates most of the country's electricity, through hydroelectric lakes on the rivers that drain the ranges. On the dry eastern side the rivers spread into wide braided channels, the Wairau, the Waimakariri and the Rangitata among them, across gravel plains built from glacial debris. The Waikato in the North Island runs for 425 kilometres, and the country's rivers together run to more than 180,000 kilometres. There are 3,820 lakes larger than a hectare, many of them dammed or regulated as hydroelectric reservoirs; Lake Benmore was created for that purpose. Rivers, lakes and ponds cover about 1.6% of the land area by the Land Cover Database figures, and six wetlands covering almost 551 square kilometres are listed as Ramsar sites of international importance, the Whangamarino Wetland among them.

How earthquakes and floods reach the towns

New Zealand records around 14,000 earthquakes a year. The Wairarapa earthquake of 1855, which Te Ara gives as magnitude 8.2 and Wikipedia as 8.3, shifted land up to 18 metres along the Wairarapa Fault and lifted parts of the coast more than 6 metres. An earthquake in 1931 devastated Napier and Hastings. A sequence of shallow earthquakes directly beneath Christchurch from 2010 killed 185 people, brought down buildings and liquefied the ground under the city.

Flooding is the hazard that comes most often. Settlements sit close to hill country, where rain falls far heavier than on the lowlands, and mountain streams rise fast enough to bury farms in silt. Stopbanks, dams, forecasting and reforestation of hill country have reduced the damage. Drought comes mainly to Otago and the Canterbury Plains between January and April. Papa, the soft mudstone that makes up many North Island hills, slips readily in downpours wherever forest has been cleared from steep slopes for pasture.

Where people and farms sit on the land

Over three-quarters of the population lives in the North Island, half of it north of Lake Rotorua and a third of it in the Auckland Region. The South Island holds a little under a quarter. Britannica gives the urban share of the population as 83.7% in 2018, and four cities, Auckland, Christchurch, Wellington and Hamilton, hold the largest urban populations. The country is divided into sixteen regions, nine in the North Island and seven in the South, with boundaries drawn largely along drainage basins.

About two-thirds of the land is economically useful and the rest is mountain. In 2007 almost 55% of the land area was farmed, three-quarters of it as pasture for sheep, cattle and deer. Arable land is 1.76% of the total. Taranaki's mild climate supports dairy farming up the slopes of its volcano, and the river terraces north of Lake Taupō carry dairy and mixed farms. North of Auckland, Northland becomes gradually subtropical, its coast cut by deep inlets fringed with mangrove swamp. The Canterbury Plains, the alluvial fans of the eastern South Island rivers, are the one extensive lowland in the South Island, and Central Otago, a high plateau behind the ranges, has a climate close to continental.

What is left of the forest and the birds

The islands evolved in isolation for 80 million years. Apart from a few bats there were no native land mammals, and birds filled their place: the kiwi, the kākāpō, the takahē and the weka cannot fly. The moa were hunted out by about 1500. Fire after Polynesian settlement took about 40% of the original forest by one Wikipedia account and around half by another, European settlers logged or cleared much of what remained for pasture, and forest now covers 23% of the land. Te Ara puts the total removed by burning and farming at over two-thirds of the native forest. Around 30 bird species are listed as endangered or critically endangered, and many survive on offshore islands cleared of predators.

UNESCO lists three World Heritage sites: Te Wahipounamu in the south-west, inscribed in 1990; Tongariro National Park, inscribed in 1990 and extended in 1993, which is listed as a mixed site for its cultural as well as its natural value; and the Sub-Antarctic Islands, inscribed in 1998. The outlying Kermadec and Subantarctic islands are inhabited only by small numbers of conservation staff. New Zealand has no land border, and the Ross Dependency, its claim in Antarctica, is one that most countries do not recognise.