Parents need to know their child's weight status and help them become healthy if necessary, say experts.
Many parents misjudge their child's weight which can result in them doing nothing to help their kids become healthy.
While 80.9 per cent perceived their child's weight as normal, 71.8 per cent of them were actually a healthy weight based on BMI (body mass index), research published in the Australian and New Zealand Journal of Public Health shows.
And 8.2 per cent and 0.2 per cent perceived their child to be overweight or very overweight respectively, while the actual numbers were 16.3 per cent and 5.8 per cent.
Data was collected from 4437 parents from 2009 to 2012 as part of the Western Australian Health and Wellbeing Surveillance System.
Parents were asked if their child was underweight, normal weight, overweight or very overweight, and their intentions in relation to their weight.
BMI values were established after parents gave the height and weight of the child.
More than half the parents with children above or below the healthy BMI range reported an intention to do nothing about their child's weight.
Co-author Dr Christina Pollard, from Curtin University, told AAP if parents are wrongly classifying the weight, they are less likely to intervene.
"We are encouraging people to accurately have their child's weight assessed and then to prevent them from becoming overweight and obese."
She noted that as more children are becoming heavier in the general population it becomes "normalised", so parents may misjudge the weight category of their own kids.
"Managing and maintaining a healthy weight in childhood is really important going forward into adulthood," she said.
All parents who said their child was very overweight or obese intended to help the child lose weight, while 60 per cent of those who said their child was overweight intended to help.
Only one-third of parents who perceived their child as underweight intended to help them gain weight.