Statistics New Zealand says Auckland's population may rise to two million by early next decade.
Auckland's population is about 1.7m, home to just over one-third of the country's population (34 percent) and by 2048, it could make up 37 percent.
"Auckland will likely have the highest average annual growth of New Zealand's 16 regions over the next 30 years, from net migration and natural increase (more births than deaths) in relatively equal shares," population estimates and projections manager Hamish Slack said in a statement.
Auckland may have two million residents by the early 2030s, but the milestone may come earlier or later depending on levels of migration over the coming years, according to Statistics New Zealand.
The supercity will account for half of New Zealand's population growth over the next 30 years under Stats NZ's medium projection. This is similar to its share of growth over the last 30 years.
Auckland reached a population of one million in the early 1990s and New Zealand's population reached 5m in March 2020.
Ageing population projected to slow NZ growth in long term
The rate of population growth is expected to slow in the long term due to an ageing population.
"An ageing population means we can expect more deaths, despite increasing life expectancy, so population growth will generally slow in the long term," Slack said.
All regions are projected to have natural population decreases by the late 2040s, with the exceptions of Auckland, Waikato, Gisborne, Wellington and Canterbury.
Slack said the number of people living in each area in the future is uncertain, but every area would have more older people.
Stats NZ medium projections indicate that the West Coast is the only region that may have a lower population in 2048 than in 2018, likely due to more deaths than births and low net migration.
The West Coast's population, which was 32,400 last year, has dropped before. It was down to 31,100 in 2001 and 2002, but climbed again in 2011 and 2012 to 33,100 people.