The report from the Baker Heart Institute found that 70 percent of men and 60 percent of women aged 45-65 had a body mass index of 25 or more, meaning they were overweight or obese.
Titled "Australia's Future Fat Bomb," the study compiled the results of height and weight checks carried out on 14,000 adult Australians in 2005.
The institute's head of preventative cardiology professor Simon Stewart said the results meant Australia probably had the highest rate of obesity in the world, outweighing even the United States.
"As we send our athletes off to the Olympics let's reflect on the fact that we would win the gold medal problem now in the world fat Olympics if there was such a thing," he said.
Stewart said obesity was the major threat to Australia's future health, with an estimated nine million of the 21 million population obese or overweight.
"That is a whole million more obese adults than we had thought," he said.
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The study predicted there would be an extra 700,000 heart-related hospital admissions in the next 20 years due to obesity and almost 125,000 people would die because of the condition in that period.
The report calls for a national weightloss strategy on the scale of smoking and skin cancer campaigns, including subsidising gym memberships and personal training sessions.
It suggested hospital waiting lists could be prioritised on the basis of weightloss, to give obese people incentive to slim down.
"These are some of the controversial things we need to deal with because the healthcare system is going to be overwhelmed by weight-related hospitalisations from knee replacements through to heart attacks and strokes," Stewart said.
The report was submitted to a federal government inquiry into obesity.
