AUSTRALIA has overtaken the United States as the world's most obese nation, a new report says.

The report, Australia's Future Fat Bomb, says 26 per cent of adult Australians - almost 4 million people - are now obese, 1 million more than the last calculation in 1999.

The findings mean Australia is the world's most overweight nation, ahead of the notoriously super-sized Americans, who have a 25 per cent obesity rate.