New Zealand's population is predicted to hit 5 million in about 2020 and could reach this milestone sooner.

In the year to June 2016, the population grew by 97000, its fastest rate since the 1960s.

Statistics NZ senior demographer Kim Dunstan said both immigration numbers and population growth had exceeded their predication.

"Our population was estimated to be 4.69 million at 30 June 2016, with net migration being 69,100 over the June year."

The latest figures made it likely that the population would rise to between 4.9 and 5.1 million by 2020.

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By 2025 the population could rise by a further half a million to 5.5 million.

Based on an average migration of 30,000 a year and much higher fertility rates, New Zealand Statistics predicts that the population could grow to nearly 8 million by 2068.

If there was no migration, the population would peak at 5.3 million around 2050 and then slowly decline.

The projections also show growing numbers of older people in coming decades.

The number of people aged 85 years and older will more than triple.

Currently at 83,000, the number could rise to 320,000 in the next 30 years.

Those aged 65 years and older will roughly double, from about 700,000 now to between 1.3 and 1.5 million in 2046.

Overall population growth is expected to slow in the longer term as the population ages and the gap between births and deaths narrows. The rate of population growth may halve to less than 1 per cent in the 2030s.

NZ Statistics said the predictions were not exact forecasts and their purpose was to help with planning.