The waistlines of adult Australians, and particularly women, have ballooned in the past 30 years, research shows.
The findings have raised doubts about the accuracy of the way obesity is traditionally monitored and categorised.
The Deakin University study published in the journal Preventative Medicine found the average waist circumference of women in 2011-12 was 6.7cm bigger than for women of the same body weight and height in 1989.
And male waistlines were wider by 2.8cm, over the same period.
"Waist circumference grew significantly more than would be expected, even when taking into account increases in weight over the same period, about 5.4 kilograms for women and seven kilos for men," lead researcher Emma Gearon said.
The best explanation was that people were increasingly carrying more fat and less muscle.
Currently, medical professionals rely on the body mass index (BMI) - a calculation of a person's weight compared to their height - to monitor obesity rates in Australia.
"But this study shows that the proportion of individuals who are not obese according to their BMI - but are obese according to their waist circumference - has increased dramatically over time," Ms Gearon, who's a research fellow at Deakin's Global Obesity Centre and PhD candidate at Monash University, said.
The phenomenon was most pronounced for adult women.
The 2011-12 data shows one in 10 women were classified as being in the healthy weight range, according to their BMI. But their waist circumference indicated they were obese.
For the men, a quarter classified as overweight based on their BMI. However, when their waist circumference was measured, they classified as obese.
"BMI is quick, affordable and easy to monitor at a population level," Ms Gearon said.
"But this study demonstrates it's not effective on its own, and it needs to be re-evaluated.
"By relying on BMI alone we're severely underestimating the burden of disease associated with obesity."
These diseases include high blood pressure, type 2 diabetes and cardiovascular disease.
Ms Gearon has called for further studies to find the best way to detect and monitor obesity levels in Australia.
