At the moment, according to the Stats NZ population clock, the number is closing in on 4,934,000.
Stats NZ senior demographer Kim Dunstan said given current trends, New Zealand's population could reach the five million milestone by the end of 2019, or early 2020.
Based on forecasts around births, deaths and net migration, Stats NZ estimates that the country's population increases by one person every five minutes and 26 seconds.
The 4,000,000 milestone was reached back in 2003, but Mr Dunstan said that over the last few years, the population has grown particularly quickly.
In the year ending September 2018 the population had grown by 90,000, he said, but net migration was trending down and the population estimates would be revised once the final Census results were out.
"There are these year-to-year fluctuations in the components of population growth, so in the last year our population grew at 1.9 percent, but it was only as far back as 2012 that the population grew at 0.5 percent over the year.
"It was growing as fast as 2.1 percent in 2016 and 2017."
However, when overseas visitor numbers are taken into account, in a measure of how many people are physically in the country at a particular point in time, Mr Dunstan said the five million mark has probably already been hit.
"In the summer months, that is when we have relatively high numbers of visitors from overseas and relatively few New Zealanders overseas, hence the number of people physically in New Zealand tends to peak in those summer months."
New Zealand's population reached one million in 1908. The second million was reached in 1952.
The population hit three million in 1973 and four million in 2003.