THE OBESITY epidemic may be worse than previously thought with new research showing men and women underestimate their weight and add centimetres to their height when answering health surveys.

While the last national population survey found 54 per cent of people to be overweight or obese, the true figure is probably 66 per cent, researchers at the University of Sydney say.

By comparing self-reported data to accurately measured data for 8435 adults in the National Health Survey and National Nutrition Survey, they found 28 per cent of women and 27 per cent of men put themselves in the wrong body mass category.

Women calculated their BMI an average 1.3 units lower than their correct score, and men by 1.1 units, the report in the Australian And New Zealand Journal Of Public Health said.

A man reporting his height to be 180 centimetres is often about 2.3 centimetres shorter, and older people overestimate their height to a greater extent than younger people. Similarly under-reporting of weight and BMI was greatest for men and women over 60.

The Professor of Population Health and director of the WHO Collaborating Centre for Obesity Prevention at Deakin University in Victoria, Boyd Swinburn, said the inherent problems in asking people to supply their own figures had forced governments to try different ways to get the weight-loss message across.

He said the current "Measure Up" campaign asking Australians to measure their waistline was aimed at reducing a pervasive level of self-denial. The advertisements warn that a waist measurement of more than 94 centimetres for men and 80 centimetres for women is an indicator of the internal fat deposits, which coat the heart, kidneys, liver and pancreas, and increase the risk of chronic disease such as cancer and diabetes.

"It's human nature to paint a good picture for ourselves," Professor Swinburn said. "When you ask human beings they will underestimate their weight, how much alcohol they drink and how much they eat, and they'll overestimate their height and how much sex they have."

Led by Alison Hayes at the University of Sydney's School of Public Health, the researchers devised equations to correct self-reported height and weight in population-level surveys.

When these equations were applied to the latest survey, the prevalence of overweight and obesity in men was 75.5 per cent, compared to the self-reported figure of 63.2 per cent. For women, the corrected figure was 57.2 per cent instead of 45.5 per cent.

A second study by the School of Public Health found a high concentration of advertisements for alcohol and junk food near primary schools. Almost 2290 food ads were spotted within a 500-metre radius of 40 schools in Sydney and Wollongong. About 80 per cent were for food high in calories and low in nutrients.

The researchers, from the NSW Centre for Overweight and Obesity, called for geographic marketing bans for certain products, similar to advertising "green zones" implemented in the US city of Boston and in South Korea.