The total fertility rate has been below that replacement rate since 1980 apart from an earlier four-year baby blip around 1990 when it peaked at 2.18, and the past five years when it peaked at 2.18 again in 2008.
Professor Jackson said both blips could be traced to the ripple effects of the huge "baby boom" in the 25 years after World War II, when the total fertility rate peaked at 4.31 in 1961.
The first blip around 1990 was because of the bulk of the baby boomers born around 1960 having children 30 years later.
The latest blip is the result of women born towards the tail end of the postwar boom putting off their own child-bearing until later in life.
"The children of the baby boomers are now largely in their 30s and 40s, and it is their peak child-bearing years - and catch-up on delayed births - that is delivering the current baby blip," she said.
But that blip is ending as the last of the baby boomers' daughters to have children finally reach the end of their child-bearing years.
She said the drop in the birth rate in the past year was also partly because of the current record exodus of New Zealanders to Australia.
"For the first time since as far back as I can look in this data by age, going back into the 1990s, the net loss was people of reproductive age and their children," she said.
"Normally it's people aged 20 to 29, but in the last year we have lost people of parenting ages and their children." This was partly because of families fleeing Christchurch after the earthquake.
There was a net outflow overseas of 3063 people from Christchurch and its neighbouring Waimakariri and Selwyn districts last year, outweighing a small net inflow of 1208 overseas migrants into the rest of the country.
Combining the births, deaths and migration effects, New Zealand's population grew by only 29,600, or 0.7 per cent, the smallest increase since 2000.
Professor Jackson, who worked for 11 years in Tasmania before returning to New Zealand recently, said New Zealand faced similar problems to Tasmania, which has struggled to attract enough migrants to offset slowing natural population growth.
"We are heading that way because of that bite in our age structure [between the ages of 25 and 40]," she said. New Zealanders in the reproductive age bracket have been heading overseas, mainly to Australia, for better pay.
"In the medium to longer term, immigration doesn't do much at all to resolve population ageing because migrants also grow old," she said.
"You really would want to target young families and look closely at why so many left in the last year."
