Wellington, Dec 8 (IANS): New Zealand's population could hit the 6-million milestone by 2050 depending on migration and birth rates in the country, the statistics department Stats NZ said on Tuesday.

The department has released projections of the population usually living in New Zealand over the period from 2020 to 2073, estimating a resident population of 5.1 million on June 30, 2020 as a base, reports Xinhua news agency.

"The New Zealand population hit 4 million people in 2003," population insights senior manager Brooke Theyers said in a statement.

The manager added that it took another 16 years to reach 5 million, but lower projected birth rates and an ageing population mean the population could hit 6 million in about 20-30 years.

The latest projections indicate the population is likely to be between 5.1 and 5.5 million by 2025, and between 5.2 and 5.9 million by 2030.

By 2073, the projections indicate a population of between 5.3 and 8.5 million, Theyers said.

One in five people will be aged over 65 by the 2030s, he said, and the percentage could increase to nearly one in four (23 per cent or 1.5 million people) by 2050.