The economic advantages that Auckland and some other North Island urban centres gained in the late 19th century were further advanced in the first half of the 20th century.

Although New Zealand's exports were still largely agricultural, much of their processing took place in urban areas. The number of sheep in the North Island surpassed the number in the South Island soon after the human population had done the same, and the number of cattle in the North Island was always much higher.

The large Southdown and Westfield meatworks were located in Auckland, and other food processing plants were also significant employers here and in other North Island towns and cities.

Starting in the 19th century, but most rapidly increasing after World War II, manufacturing became an important part of the Auckland economy.

By the 1970s most types of manufacturing were one-and-a-half to two times as concentrated in Auckland as might be expected according to the size of the labour force.

More recently, it has been the growth of the service sectors which have generated rapid economic and associated population growth. Auckland is the location of many corporate headquarters, generating employment in high level financial, legal and information and communication technology
sectors.

It also has a high concentration of jobs in other service sectors such as hotels and restaurants, real estate, creative industries and education.

Auckland's ongoing rapid growth can also partly be attributed to its role as a "gateway" city, similar to other cities on the Pacific Rim such as Vancouver, Sydney and San Francisco.

In the first half of the 20th century, the port of Auckland served as the entry point for many British (and a few other) migrants as well as the export port for large quantities of New Zealand's agricultural
products.

In the 1930s the first flying boats flew out of Mechanics Bay in Auckland on commercially scheduled return flights through the Pacific to North America, and to Australia.

The subsequent development of Ardmore and then Mangere international airports made Auckland the primary entry point for migrants, business travellers and tourists, as well as air cargo.

After World War II, immigrants from the Pacific islands mostly arrived
in Auckland, initially by ship and later by air. Many first settled
in the central city which had a rapidly growing Pacific population,
though later most concentrated in the south and west of Auckland. As a result, about three-quarters of the Pacific population of New Zealand currently live in the Auckland region.

More recent migrants have also tended to land and settle in Auckland, especially after the change of immigration policy in 1987. In recent decades two thirds of all Asian immigrants have settled in Auckland, where they have been part of the transformation of the economy and
society.

The Drift North was not just a result of the growth of Auckland,
but also a result of the urbanisation happening throughout New Zealand. Interestingly, the population growth rates of Christchurch and Wellington have been remarkably similar through the 20th century, so this does not explain the drift.

However, through much of the 20th century the medium and smaller urban centres of the North Island have tended to grow more rapidly than those of the South Island.

For example, at the end of the 19th century, Hamilton was the 21st largest urban centre in New Zealand, but by the 1970s it had become the fifth, and in 2006 the fourth largest centre, having a larger population than Dunedin.

Likewise, through much of the 20th century Tauranga, Whangarei, and Napier-Hastings grew more quickly than Nelson, Oamaru and Invercargill.

There is no single explanation for these different rates of growth, but in general they relate to the more rapid expansion of various economic sectors in the north including dairying, forestry and manufacturing.

More recently, horticulture and the development of the wine industry have also been significant, although these have had a significant impact in parts of the South Island as well.

The Drift North has not been a simple one-way movement. After the 2001 census, some South Islanders celebrated the fact that, although the population of the North Island was, by then, three times as large as the South Island, and Auckland's population was growing rapidly, in terms of internal migration the South had a net gain from the North, and
there was an overall net loss from Auckland.

The net loss was shown to have increased at the next census with for example, about 1800 more people moving from Auckland to Otago than the other way around, probably including many looking for a change in lifestyle as well as employment opportunities in tourism and winemaking. In the same period there was also a net loss of 1600 from Auckland to Canterbury.

There is also recent evidence that although the majority of new
immigrants initially settle in Auckland, many are eventually moving
on to other regions, in both the North and South Islands.

Dr Wardlow Friesen is a senior lecturer in geography in the School of Environment at the University of Auckland.