Why New Zealand's population might soon be in decline

The world's population is closing in on 8 billion, but most of the growth is happening in parts of the world that can't afford it, experts say.

Friday is World Population Day. It's been 30 years since the first - since then, the population of the world has increased by almost half - from 5.2 billion to 7.7 billion. 

"We've added 6 billion to the world's population in the last 100 years," sociologist Paul Spoonley told The AM Show on Friday.

"There are two reasons for that - we've improved people's health, and fertility has remained high. It's not sustainable, and the UN is expecting another 2 billion in the next 30 to 40 years. It hasn't peaked yet."

It took all of human history up to the year 1803 for humanity to reach a population of 1 billion. The next billion took 124 years, but advances in medical science in the early 1900s saw the population boom.

It took only 33 years to reach 3 billion (1960), 15 years to reach 4 billion, then 12 years to reach 5 billion and 6 billion (1999).

Since then, growth has slowed - barring some sort of apocalypse, it'll take 13 years to go from 7 billion to 8 billion (2024). The UN expects growth to slow further as incomes and education levels rise in poorer parts of the world - both of which are linked to falling numbers of children being born - but in the meantime they're still expected to contribute the most in the coming decades.

"Sub-Saharan Africa will contribute most to the future population growth," said Prof Spoonley.

"They've got very high fertility and we're reducing their mortality. The combination of having between four or five children - which is what the average is for sub-Saharan Africa - and longer lives - so people aren't dying as young - is the combination for that growth."

But 55 countries are already at below replacement-level growth, most relying on immigration to keep population levels steady or growing. New Zealand's natural increase - births minus deaths, excluding immigration - is about 30,000 a year, down from more than 40,000 in the early 1960s despite a much larger population.

'We are one of the countries you'll expect to see decline," in future, said Prof Spoonley.

New Zealand's population is expected to surpass 5 million at the end of this year.

According to World Population Review, the world gains about 224,500 people per day. In the time it took you to read this far, another 300 people would have been born. By 2050, the world's population is expected to be closing in on 10 billion.

"That's something that we really have to address," said Prof Spoonley. "Adding another 2 billion is really going to put a lot of pressure on the planet."