South Western Sydney Local Health District's (SWSLHD) healthy children's initiative manager Leah Choi said small changes and extra care when packing lunches and snacks could be the key to positive change to guard against obesity in school children.

"Healthy habits formed early in life can stick with a person beyond childhood, so there's no better time to teach children healthy food habits than now," Ms Choi said. "Eating healthy food can help children concentrate at school and can give them the right type of energy for their day."

Ms Choi suggested making simple changes like making sure you pack food that comes in its own packaging, such as apples, sultanas, nuts, grapes or boiled eggs rather than packaged chips or sugar-laden muesli bars.

"Considering children often consume up to 50 per cent of their daily calories at school, it's important that their lunchbox is filled with yummy, healthy choices," she said.

Appropriate serving sizes should also be regulated.