"The medium-growth projection suggests Auckland's population could expand by an average of about 60 extra people every day, or 1900 a month."
Already with 1.7 million residents, Auckland is home to just over one-third of New Zealand's population (34 per cent). By 2048 it could make up 37 per cent of the country's population.
Auckland will account for half of New Zealand's population growth over the next 30 years, under the medium projection.
However, projections suggest that the rate of growth in all regions will slow in the long term.
Slack said despite increasing life expectancy, an ageing population means we can expect more deaths, slowing population growth.
"The total number of people who will be living in each area in the future is uncertain. However, what is certain is that every area of New Zealand will have more older people."
All regions except Auckland, Waikato, Gisborne, Wellington, and Canterbury are projected to have natural decreases by the late 2040s, with more deaths than births.
The West Coast is the only region to have a lower projected population in 2048 than in 2018.
This population dip will likely be the cause of more deaths than births and low net migration.
StatsNZ projects deaths could exceed births on the West Coast by 50 a year in the early 2030s, and by 180 a year in the late 2040s (medium projection).
The West Coast's population has dropped before, in 2001 and 2002 when it fell to 31,100. As of last year the region had a population of 32,400.
Of New Zealand's 67 territorial authority (TA) areas – or city and district council areas – 61 are projected to have more people in 2048 than in 2018.
Selwyn, west of Christchurch, and the Queenstown-Lakes districts are projected to have the fastest-growing areas.
The six TA areas expected to have smaller populations in 2048 than in 2018 are Waitomo, Ruapehu, Buller, Grey, Westland and Gore.
