A new study is predicting two million New Zealanders could be considered clinically obese in the next 20 years.

The Otago University study found that Body Mass Index (BMI) of New Zealanders is on the rise and the average BMI is on track to be above the obesity threshold by the early 2030s.

BMI is a measure of body fat based on height and weight that applies to adult men and women.

An index between 18.5kg/m2 and 25kg/m2 is considered the healthy weight range - anything at 30kg/m2 or above is considered to be obese.

Lead researcher Ross Wilson says obesity rates had tripled between 1977 and 2013.

"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."

Greater availability of high energy, low nutrient food, as well as a drop in physical activity, had all contributed to the rise in obesity, he says.

"A comprehensive obesity reduction strategy might include, among other things, improving the relative affordability of healthy foods, restrictions on marketing of unhealthy foods and promotion of active modes of travel such as walking and cycling."

Healthcare costs associated with treating obesity-related conditions in New Zealand were estimated to be $624 million in 2006.

Ross says given ongoing increases in obesity over the past decade, current costs were likely to be substantially higher than this.

Results from an annual Ministry of Health survey late last year showed that 1.2 million adults and 99,000 children aged between two to 14 were dangerously overweight.

Those figures have been increasing since 2011, with a rise of nearly 6 percent and 4 percent, respectively from 10 years ago.

Otago University Professor of medicine Jim Mann says those were terrible statistics but he was not surprised.

"My main reaction is one of continuing dismay ... the population [for obesity] is horrendous, we've got an epidemic of obesity so it's a good reminder that we've got one enormous problem in New Zealand."

Jim says New Zealand had one of the worst rates of obesity in the world for OECD countries.