New Zealand's fertility rate has now dropped below replacement levels, meaning there are not enough babies being born to replace the current population.
Demographer Paul Spoonley discusses what's going on.
New Zealand's population is growing – up 2.1 percent last year – but the contribution of fertility to that growth is declining.
Yet we're not alone - most other countries in the OECD have already "gone sub-replacement" in terms of population growth, Spoonley says.
The domestic populations of Germany and Japan have been contracting since the '70s. Portugal is contracting now too, with Spain and Italy soon to follow.
Within two or three decades, New Zealand will have a 1:1 ratio of people working and people not working.
Too little income being generated through taxes and too small a workforce to serve the population will be unsustainable, he says.
New Zealand's highest-ever fertility replacement rate was in the late 1960s – 4.3 children per woman. It's currently 1.8 children per woman.
It's time to take a closer look at why fertility is playing less of a role in providing a workforce and look at better support for women's education and working mothers, Spoonley says.
"Immigration is only one factor in terms of a population debate. Why don't we have a population debate which says 'What would encourage people to have children and not force them into a choice – 'Can I afford a child, or can I buy a house and keep a job?'"