Australian babies are being born heavier, increasing the likelihood of health problems later in life, a new study has found.

Researchers analysed records from more than a million births in New South Wales state between 1990 and 2005, finding the number of boys considered heavy increased 18 percent, with the figure rising to 21 percent for girls.

The study by researchers at Sydney's Royal North Shore Hospital warned that high birthweights, typically four kilograms (8.8 pounds) or more, were associated with increased risk of cancer, asthma and diabetes later in life.

It cited a number of reasons for the higher birthweights, including less smoking among expectant mothers and a higher rate of gestational diabetes, also noting "the prevalence of overweight or obesity among pregnant women in Australia is high".

But it said such factors did not explain the extent of the change, which reflects similar findings in Sweden and Canada, and called for more studies in the area.

The research was published on Monday in the Medical Journal of Australia.

Michael Permezel, spokesman for the Royal Australian and New Zealand College of Obstetricians and Gynaecologists, believed higher rates of obesity among pregnant women was behind the phenomenon

"It really is a community obesity thing and related to activity during pregnancy and diet during pregnancy," Permezel told Melbourne's Herald Sun newspaper.

"It is absolutely not necessarily the bigger the better. Obviously, if the smallness is due to the placenta not working properly, that is not terribly good, but being too big can be harmful as well."

More than 7.5 million of Australia's 21 million population are estimated to be overweight or obese, accounting for two-thirds of all men, half of the women and one quarter of the nation's children.

International studies consistently rank Australia among the fattest countries in the world, with the nation's Baker Heart Institute in 2008 suggesting it faced a "fat bomb" outranking even that in the United States.

Emergency services have introduced special "mega-lift" ambulances in New South Wales state capable of handling patients weighing more than 180 kilograms (397 pounds).

Undertakers also say they are stocking massive coffins, while consumer authorities are considering upgrading standards on everything from toilets to child car booster seats so they can handle heavier loads.