Two million New Zealanders will be obese in 20 years time, new research reveals.

An Otago University report published today in the Australian and New Zealand Journal of Public Health showed the average Body Mass Index (BMI) of Kiwis increased from 26.4 in 1997 to 28.3 in 2015.

If the trend continued, New Zealand's average BMI would exceed the obesity threshold of 30 by the early 2030s.

In 2015, 1.1 million New Zealanders were considered clinically obese but by 2038 that could be two million.

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The prevalence of obesity in New Zealand had already trebled to 30 per cent between 1977 and 2013, making it the third most obese nation.

Researchers Dr Ross Wilson and Professor Haxby Abbott from the University of Otago's Centre for Musculoskeletal Outcomes Research at the Dunedin School of Medicine said the Government needed to take action to address the situation.

"High BMI has now overtaken tobacco as the greatest contributor to health loss in New Zealand, which emphasises the public health importance of these findings," Wilson said.

Otago University researchers Dr Ross Wilson and Professor Haxby Abbott found two million New Zealanders will be obese by 2038. Photo / Supplied
Otago University researchers Dr Ross Wilson and Professor Haxby Abbott found two million New Zealanders will be obese by 2038. Photo / Supplied

The study also showed the BMI of Māori and Pasifika and those living in socially deprived neighbourhoods was increasing at a faster rate. In 20 years the average BMI of the Pasifika population was expected to exceed the general population by 7.1-8.1 - up from a difference of 6.4 in 2015.

The research, which used data from more than 76,000 adults, found the increase in BMI was down to factors including greater availability and consumption of high-energy, low-nutrient foods and lower levels of physical activity.

"These findings emphasise the need for effective public health measures to address the causes of the obesity epidemic," the authors said.

"Altering or mitigating these environmental influences is therefore crucial to slow or reverse projected increases in population obesity."

Obesity was a risk factor for many common health conditions, including diabetes, cardiovascular disease, osteoarthritis and cancer which contributed the loss of quality of life, early death and higher healthcare expenditure, the report said.

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"High BMI is the greatest contributing risk factor to health loss in NZ. Healthcare costs associated with treating overweight- and obesity-related conditions in NZ were estimated to be NZ$624 million in 2006, representing 4.4 per cent of all healthcare spending."

The researchers suggested tobacco control could be a useful parallel for considering the importance of comprehensive reforms across a range of policy areas in halting the spread of a public health epidemic like obesity.

They suggested improving the relative affordability of healthy foods; restrictions on marketing of unhealthy foods; better nutritional labelling; more education; promotion of active modes of travel; and provision of facilities and space for physical recreation could all help stem the country's growing waistlines.