New data from the Australian Heart Foundation shows the weight of the average Australian adult has increased by more than 5.7kgs over the past 25 years.

The foundation says the average weight of an Australia man is now 85.9kg, up 6.5kg, while the average woman is 5.7kg heavier at 71.1kg.

It has used the figures to warn of a rapid spike in diabetes, heart disease and cancer, unless growing rates of obesity are brought under control.

Robert Grenfell from the Heart Foundation says it is a major concern that two in three people fall outside the healthy weight range.

"Every two in three Australians is now either overweight or obese," he said.

"Now this, of course, is alarming because the diseases that are a consequence of this [include] diabetes, heart disease and some cancers.

"We're going to see a rapid increase in these diseases over the next 10 to 20 years if nothing happens."

Dr Grenfell says the proportion of obese adults has tripled since 1980.

"We've nearly got half a million Australians and they are what we call morbidly obese and that is a BMI of over 40 and they are at extreme risk of an early death from any of those diseases," he said.

"But again from 1980, where 60 per cent were at normal, healthy weight, we now only have 35 per cent of Australians.

"This of course is a message that we really need to do something about this."