Massey University sociologist Paul Spoonley joins Mike Hosking to talk about new Zealand's rise in population. The country cracked 5 million people yesterday.

It took just 17 years to add one million people after hitting 4 million in 2003. About half that million came from natural increase (births minus deaths) with the other half coming from net migration.

On average, the population has grown by 1.8 per cent a year since 2013, driven by net migration.

Although it was still a provisional milestone, hitting 5 million was a "significant event" for New Zealand, Stats NZ's population insights senior manager Brooke Theyers said.

The Covid-19 pandemic had caused unusual international travel and migration patterns in recent months, which had boosted net migration as more Kiwis arrived home from overseas, she said.

"At the same time, New Zealand citizens may have been unable or reluctant to head offshore."

Mr Spoonley believes with everything that is currently going on, the population will fall from here.

"Our fertility has been dropping quite steadily.

"I would think the pandemic will see another dip in births in nine months time, and what you're left with is no migration."

He adds per hectare New Zealand is very underpopulated.

Population estimates will need to be revised to incorporate all 2018 Census data, which will happen later this year. International migration estimates will also need revision - especially if recent migrant arrivals head back overseas when border restrictions are relaxed and travel resumes.

Stats NZ conceded they couldn't tell whether our 5 millionth person was a new migrant who arrived in New Zealand on a plane, or if it was a newborn baby. The precise date when we hit 5 million also isn't available yet.

But it's estimated that at March 31 had a resident population of 5,002,100, Stats NZ said.