New Zealand's population is expected to hit 4.6 million at about 6.45pm tonight.
The Statistics New Zealand Population Clock uses census data and other information to predict the population.
It accounts for one birth every nine minutes and two seconds, one death every 17 minutes and 32 seconds, and a net migration gain every 11 minutes and 17 seconds.
The clock forecasts are based on recent trends but Kim Dunstan, a Statistics New Zealand senior demographer, said the counter clock is a good indication of how the population is changing.
"Our population clock continues to tick over, so that population clock is based on our latest estimate which is published for the end of March this year.
"We apply some forecasts of births, deaths and net migration - the three things that can change a population."
Mr Dunstan said more official population estimates to the end of June will be published next month.
He said net migration, which is the difference between arrivals and departures, is the main driver of New Zealand's population growth.
"So in the year to the end of May, net migration was 58,000 and natural increase, which is the difference between births and deaths, is running at about 27,000 a year.
"Clearly net migration is currently making the largest contribution to our population growth which is a bit of a change from the long-term historical trend where natural increase has been the main contributor," he said.
Mr Dunstan said long-term population projections showed a "general slowing" of population growth as the population ages, because of more deaths and fewer births.
"Net migration has always been quite volatile in New Zealand, it was only a few years ago we had more departures than arrivals.
"The projections do indicate we can expect ups and downs in our migration balance to continue," he said.