There are up to 18,000 people per square kilometre in Central Auckland.

If the country was scaled to size based on population, the North Island would dwarf the South Island.

Using the technique is an effective way to demonstrate the increasing difference in population on the two islands.

At June 30 this year, the North Island was home to 77 per cent of the population, compared to the 23 per cent on South Island, according to Stats NZ.

That means there was an estimated 3.67 million people in the north, and an estimated 1.1 million in the south.

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While the South Island population grew by 1.8 per cent in 2017, it's still smaller than Auckland, and the North Island's population grew by 2.2 per cent in the same period.

Cities and neighbouring districts grew the fastest.

Figures showed the country's population density was 18 people per square kilometre - up from 13 in 1991.

However, density varied hugely depending on the area - central Auckland had up to 18,000 per square kilometre.

Stats NZ also calculated the country's median centre of population.

That was defined as "the point of intersection of two lines: one equally dividing the population north and south, the other equally dividing the population east and west."

It's now about 1km offshore, west of Kawhia in the Waikato.

It has moved 280km north since 1921, when it was located offshore, north-west of Kapiti Island.