New Zealand's population, which more than trebled last century to 3.8 million, is forecast to peak at 4.6 million in 44 years' time before falling to 4.2 million by 2101.
The patter of tiny feet will be replaced by the creak of ageing bones as the numbers of childrenfall and those aged over 65 more than double by mid-century.
By then one in four people will be aged over 65. Of those a quarter will be aged over 85 - including an estimated 12,000 centenarians.
Overall, the median age will rise from 34 to 45 by the 2040s, still younger than many developed countries. Our workforce will also be smaller and older.
Statistics New Zealand released its first 100-year population projections yesterday as a benchmark for policymakers.
It highlighted a medium path from eight models with different fertility, migration and life expectancy figures. That forecast the country will reach 4 million people in 2006, rising to a peak of 4.6 million in 2044.
After that the impact of lower fertility rates - a slide which started in the 1960s - will help send the population into decline. Increasing deaths as baby boomers hit old age will start exceeding births and the estimated gains from migration.
The medium forecasts are based on a model assuming a 5000 net gain from migration each year - the average gain last century. In the wildly fluctuating 1990s the average migration gain was 10,000 but in the past year there has been a 10,000 net loss.
The projections also assume 1.9 births a woman. The present rate is two.
Despite its decline New Zealand's birth rate is still relatively high compared with other developed countries, largely due to the higher fertility of Maori and Pacific Island women. Italy is already down to 1.2.
The third assumption in the medium model is a seven-year increased life expectancy for males to 82, and an extra six years for women to 86.5.
On Monday, the Herald starts a series, looking at the role of immigration in New Zealand - who comes and why, the challenges they face and the problems attracting migrants.
