CANBERRA: Australia is on track to becoming the fattest nation, a study has showed, although experts questioned on Friday whether it had overtaken the United States and small Pacific countries for the unenviable title.

About 4 million Australian adults, or 26 percent of the population, were obese, eclipsing the 25 percent rate in the US, the study by the Baker Heart and Diabetes Institute released in Melbourne said.

"If we ran a fat Olympics we'd be gold medal winners as the fattest people on earth at the moment," the Institute's preventative cardiology head, Professor Simon Stewart, told The Age newspaper.

The report, titled "Australia's Future Fat Bomb", to be presented to a government inquiry into the nation's obesity problem on Friday, said 70 percent of men and 60 percent of women aged 45 to 65 were technically overweight or obese.

In total, 9 million people were "too heavy" - almost half the 21 million population - and 123,000 were at risk of early death over the next two decades, the study said.

While the report said Australia had overtaken the US as the fattest nation, recent US studies showed that about 34 percent of Americans are overweight or obese.

Small Pacific nations also top World Health Organization (WHO) lists, with 94.5 percent of people in tiny Nauru classed as overweight, leading to chronic diabetes problems on the island.

The Federated States of Micronesia (91.1 percent), the Cook Islands (90.9 percent), Tonga (90.8 percent) and Niue (81.7 percent) rounded out the WHO top five, while the US came in 9th, with 74.1 percent overweight or obese.

Australia came in at 21, behind countries such as New Zealand, Mexico, Greece and Kuwait, the WHO reported.

In all, there are currently 1.6 billion overweight adults in the world, a number that is expected to grow by 40 percent over the next decade, the WHO said.

Nutrition expert Rosemary Stanton said that while Australia may not lead the world, it did need to take urgent action to address growing obesity and rethink failed health messages.