Growth in Christchurch has reversed sharply since the February earthquake, and the city has had a net loss of 8900 people in the year to the end of June.
"The population of most of our non-urban areas will not grow any more in any significant manner," Professor Jackson said.
"It's a very hard message for a lot of town councils and regional councils to cope with."
Her study, to be presented at a Population Association conference on Monday, warns New Zealanders not to be complacent because their country has one of the developed world's highest overall rates of population increase, with a high fertility rate and - in most years - a net migration inflow.
Despite these facts, New Zealand's age structure has a huge "bite" out of the middle in the 25 to 39 age group, where there are about 80,000 fewer people than in the comparable 15-year age brackets immediately older (40 to 54) or younger (10 to 24).
Professor Jackson said this was due to the coincidence of two roughly equally important factors - a dramatic decline in the fertility rate in the 20 years after about 1960, and a net outflow of young adult New Zealanders to Australia in the past 20 years.
This means fewer people aged 25 to 39 were born and fewer of them remain in the country. The gap affects both population growth and the economy.
"People aged 25 to 39 are the people who have kids," she said.
Auckland still has plenty of them - the "bite" that shows up in the national age structure is barely discernible in the region because of an influx of young adults from other regions and from overseas.
The lack of people in the breeding age group is extreme in rural districts such as Matamata-Piako, Thames-Coromandel, Hastings and Gore.
"The picture for the regional areas is rather gloomy in that they are going to have fewer young people," Professor Jackson said.
The gap will also have an economic impact because young adults are part of the prime working age group.
The study says 42 per cent of local body areas have fewer people in the 15-24 age group coming into the workforce than they have in the 55-64 group heading for retirement.
"If just a small proportion of the current 15-19 year cohort leaves New Zealand and doesn't return, New Zealand employers will be faced with a labour shortage of crisis proportions,"she warned.
"We are not talking about 20 years hence when new technology may require fewer workers, but rather a situation that has already begun, is significant outside of the main centres, and will become painfully evident within the next five years."
She said this could have advantages for workers, as unemployment was likely to drop and wages were likely to be pushed up.
But it posed challenges for businesses, especially in areas such as farming where most farmers in some regions were nearing retirement.
Professor Jackson believes it will be crucial to invest in education and training to give young people the skills to replace retiring workers and to earn incomes high enough to keep them in New Zealand.
Maori poised to get baby-boomer jobs
Maori stand to be the big winners from the end of population growth in most parts of New Zealand in the next 20 years or so.
Waikato University Professor Natalie Jackson will tell a Population Association conference in Auckland on Monday that Maori could reap a "demographic dividend" because they are still a youthful population at a time when the workforce available in most parts of the country will soon start to shrink.
"Instead of the doom and gloom and the idea that young Maori are going to be unemployed like their parents, here is an opportunity," she said.
"A massive wave of baby-boomers are going to retire over the next five to 10 years, and young Maori are poised to move into the labour market to mop up some of those jobs."
Children under 15 make up 34 per cent of the Maori population compared with only 19 per cent of European New Zealanders.
Put the other way around, that means 26 per cent of all children under 15 are Maori, compared with only 13 per cent of adults aged 15-plus.
"There will be many opportunities for Maori as the older (predominantly European-origin) population retires and is replaced by ever-smaller cohorts," Professor Jackson said.
"A key need for Maori will be to identify those opportunities and to ensure that Maori of all ages are trained appropriately.
"What it requires is for the message to get out there so that young people can see the potential value of staying on and getting an extra qualification."
