Obesity even at a very young age is costing the health system money, with obese pre-schoolers two to three times more likely to be admitted to hospital, a study has found.

The study by the University of Sydney's School of Public Health examined the total health care costs of 350 pre-school aged children over a three-year period and found those who were obese had 60 per cent higher costs than healthy weight children.

There was no difference between overweight children and healthy weight children.

Examining the Medicare, medication and hospital records of these children, it was also found obese children aged under five were far more likely to be hospitalised for common respiratory disorders, such as asthma, and diseases of the ear, nose, mouth and throat.

Health economist and lead researcher Associate Professor Alison Hayes says the study, published in the Obesity journal, highlights the economic burden of childhood obesity.

"People haven't really looked at this very young age group in much detail before but what our study is showing is even at this very early age obesity is actually costing money," she told AAP.

The cost of obesity in pre-schoolers is estimated to be around $17 million per year, she said.

"We know that children who are obese in early childhood are more likely to be obese in later childhood, adolescence and adulthood, which can lead to serious chronic diseases that have a huge impact on our health care system.

A report card on the nation's health released this week showed that Australia is among the fattest nations in the world.

Statistics show nearly one in four children in Australia is overweight or obese before they start school.

With kids eating too much high-calorie food and not doing enough physical activity, Dr Michael Gannon, president of the Australian Medical Association, says it's time to invest in the future to "reap the benefits in the years to come".

For parents, who find it hard to make healthy choices when energy-rich foods are available 24/7, dieticians say its about the basics.

Dietitian Melanie McGrice from the Dietitians Association says children need to eat wholefoods rather than processed foods.

"It is difficult for mums to find the time to prepare these foods but its going to be a much safer option.

"It's cheaper as well," she said.