New Zealand's population has grown by nearly 450,000 people - the equivalent of the Waikato region - in the past five years and is closing in on five million people.

Statistics New Zealand said today that the estimated resident population was 4,885,300 on June 30 - an increase of 1.9 per cent on the previous year.

That was slightly slower growth than in the year to June 2017, when the population grew by 2.1 per cent - partly because fewer migrants are arriving in New Zealand and more are leaving.

This country's growth is high by historical and international standards. Australia's population rose by 1.6 per cent in the past year, and the global population rose by 1.2 per cent.

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The main driver behind the growing population is net migration, which reached a peak of 72,400 in the July 2017 year. Natural increases in population (births minus deaths) contributed around 30,000 more people a year.

Since the census in 2013, the New Zealand population has grown by 443,200. It is expected to reach the 5 million milestone around 2020, but that could come earlier if net migration remains high.

The population is growing in most age groups. In the over-30 group, women outnumber men, while in the under-30 group, men outnumber women.

There are 14,600 more men than women in the 20 to 24-year old age group, and 14,000 more women than men in the 45 to 49 year-old age group.

Stats NZ said there were a number of reasons for this disparity, including migration trends, sex ratios at birth, and women living longer than men.

The median age of males and females has grown to more than 2.5 years, compared to just one years a decade ago. The median age for New Zealand men is 35.6 years and for women it is 38.2 years.