A new study predicts New Zealand will reach peak population in 2095 — with net migration maintaining the population as the country faces a decline in fertility from this decade.
The report published in The Lancet projects the world's population will peak at 9.73 billion in 2064 before falling back to 8.79 billion in 2100.
The University of Washington's Institute for Health Metrics and Evaluation's findings showed that global fertility rates will nearly halve from 2.4 in 2017 to 1.7 in 2100 – largely due to factors such as the availability of contraception and women in education and careers.
Countries including Japan, Spain, Italy, Thailand, Portugal, South Korea and Poland are expected to see their populations halve by the end of the century.
The results upend current expectations and suggest that "once global population decline begins, it will probably continue inexorably".
"Our findings suggest that, because of progress in female educational attainment and access to contraception contributing to declining fertility rates, continued global population growth through the century is no longer the most likely trajectory for the world's population," the authors said.
New Zealand's population is set to peak to 6.01 million in 2095, predicting the country's population growth will be sustained by net migration rather than replacement fertility rates, the authors said.
"For high-income countries with fertility rates lower than the replacement level, the most immediate solution is liberal immigration policies," the report reads.
"Among high-income countries, Canada, Australia, New Zealand, and the USA have consistently pursued this approach in the past 30 years.
"As long as these immigration policies continue, our reference scenario showed sustained population growth and workforce expansion in these countries, with concomitant economic growth.
"However, although not the case in Canada, Australia, and New Zealand, liberal immigration policies in the USA have faced a political backlash in recent years, which threatens the country's potential to sustain population and economic growth."
A map shows New Zealand's replacement population level is projected to fall in the 2020s.
Meanwhile, Australia's population is due to peak at 36.38 million in 2096.
"Our findings show that some countries with fertility lower than replacement level, such as the USA, Australia, and Canada, will probably maintain their working-age populations through net immigration.
"Our forecasts for a shrinking global population have positive implications for the environment, climate change, and food production, but possible negative implications for labour forces, economic growth, and social support systems in parts of the world with the greatest fertility declines."
Australia will also rise up the economic ranks before the end of the century from being the world's 12th biggest economy in 2017 to the eighth by 2100 putting it just behind the UK.
It states 23 countries including Japan, Spain, Italy, Thailand, Portugal, South Korea and Poland are expected to see their populations halve by the end of the century.
For example, Japan will decrease from around 128 million people in 2017 to 60 million in 2100, Thailand (71 to 35 million), Spain (46 to 23 million), Italy (61 to 31 million), Portugal (11 to 5 million), and South Korea (53 to 27 million).
The world's most populous country, China, is expected to peak at 1.4 billion people in four years before falling back almost 50 per cent to 732 million by 2100 with India becoming the largest nation.
Sub-Saharan Africa, meanwhile, will triple in size to some 3 billion people, with Nigeria alone expanding to almost 800 million in 2100.
The US is expected to retain its status as the world's largest economy until 2050 when it will be overtaken by China. However, this will then reverse, with the US the world's largest by 2100 the report states.
"By the end of the century, the world will be multipolar, with India, Nigeria, China and the United States the dominant powers," said the Lancet editor Richard Horton, describing the study as outlining "radical shifts in geopolitical power".
As fertility falls and life expectancy increases worldwide, the number of children under 5 is forecast to decline by more than 40 per cent, from 681 million in 2017 to 401 million in 2100, the study found.
At the other end of the spectrum, 2.37 billion people — more than a quarter of the global population — will be over 65 years old by then.
Those over 80 will balloon from about 140 million today to 866 million. Sharp declines in the number and proportion of the working-age population will also pose huge challenges in many countries.
– with wires, nzherald.co.nz