Australia's obesity crisis is far worse than experts thought, with new evidence showing 40 per cent of adults are dangerously fat.
The new figures are based on the waist circumference of 11,000 people who were tracked for 12 years in the Baker IDI Heart and Diabetes Institute's AusDiab study.
Previous estimates using body mass index were 25 per cent.
"The results surprised us," says Associate Professor Anna Peeters, president of the Australian and New Zealand Obesity Society.
When measured around the waist, a man should not be more than 102cm and a woman 88cm, says Prof Peeters, who will present the findings at the society's scientific meeting in Melbourne on Thursday.
She is pleased about growing government momentum against obesity, but says parents need more support.
"It is important to intervene early in life. We need good programs for parents from before their children are born."
A lot can be learnt from the US, which has decreased childhood obesity.
A minority of Australian schools adhere to dietary guidelines, she says, and workplaces should revise their catering and what is allowed in their vending machines.
Obesity is a condition of an inactive affluent society that consumes in excess, says Prof Peeters, head of obesity and population health at Baker IDI.
However, people of lower education and income who live in rural areas are most at risk because energy-dense, nutrient-poor food is relatively cheap.
"People don't need to be thinking too much about weight loss when they are a little overweight, but they do need to be thinking about preventing weight gain," she says.
"If you are a little overweight at 30, you are likely to be quite overweight by 50."
One way to avoid children growing up to be obese is to limit sugar intake, says University of Sydney Associate Professor Tim Gill, who will chair a session at the conference.
"We are burning so few calories that every calorie we eat needs to come from nutritious food."
Active children should be allowed two small treats a day at most, he says.
The biggest problem is sugary drinks, which should be limited to one glass a week.
"People are not aware how much sugar they are eating.
A can of soft drink has eight to 10 spoons of sugar. But children and teenagers typically drink double that in a serve, he says.
"Foods that contain a lot of added sugar contribute little nutrition, but a lot of calories."
Read more: We're eating ourselves to death and how to stop.