WELLINGTON - New Zealand's population is set to exceed 6 million by 2040, driven mainly by migration, according to the statistics department Stats NZ on Wednesday.

The 6-million projected population is up from 5.3 million currently, with international migration expected to remain the dominant driver of growth, Stats NZ said.

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Two-thirds of the forecast increase will come from net migration, which is the difference between arrivals and departures, while natural growth (births minus deaths) will contribute the rest, it said.

Projections show the population could reach around 7 million by 2060 and nearly 8 million by the late 2070s. However, the country is also aging rapidly. The median age is expected to climb from 38 today to the late 40s by the 2060s, according to Victoria Treliving, Stats NZ spokesperson for population statistics.

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The number of people aged 65 and over, currently about 900,000, is set to top 1 million by 2029 and double to 2 million by 2070, statistics show.

"One in six New Zealanders is now aged 65 and over. By the mid-2030s this will be one in five, and by 2060 about one in four will be in this age group," Treliving said, adding that one in 12 New Zealanders were aged 65 years and over in the mid-1960s.

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Stats NZ also modeled alternative scenarios. Without any migration, the population would peak below 5.5 million in the early 2040s before gradually shrinking. By contrast, if net migration rose to 100,000 annually, the population could reach 11 million by the early 2070s, she said.

With the current fertility rate expected to dip from 1.59 to 1.55 births per woman by 2051, population growth will increasingly rely on migration, analysts said.