Fed on a steady diet of fast food, soft drinks and television, 20 per cent of children are overweight or obese by the time they start kindergarten.
Parents are unable to recognise their children are obese, public health experts say, making it difficult to get across health promotion messages.
The University of Sydney study of more than 500 children aged up to five found that the home environment was the most significant factor contributing to their weight.
Of the overweight children studied, 30 per cent had a television in their bedrooms and nearly half ate dinner in front of the television more than three times a week.
The study leader, Louise Hardy, said it was ''horrifying'' that children were being exposed to so much television by the time they turned five.
30 per cent of the parents of obese children thought their child was the right weight ...
''Perhaps parents are upgrading their TV and don't want to toss out the old ones, so they're ending up in kids' bedrooms,'' Dr Hardy, from the university's school of public health, said.
''Kids are also being rewarded for good behaviour with sweet food. They are drinking sugar in soft drinks and fruit juices and once these negative health behaviours are established, they're very difficult to change.
''It may sound draconian, but why are we rewarding children for good behaviour at all?''
More than 60 per cent of both healthy and overweight children were rewarded for good behaviour with sweets, while more than one-fifth of overweight and obese children did not eat breakfast, found the study published in the international public health journal, Preventive Medicine.
Overweight boys were more likely to eat dinner in front of the television and watch too much of it, while overweight girls were more likely to have a television in their bedrooms and be rewarded with sweets.
Dr Hardy said it was concerning children were being introduced to bad habits so young but added that parents often did not realise their children were unhealthy.
''We asked parents whether they perceived their child to be overweight, healthy or underweight and found 70 per cent of parents of overweight kindergarten children thought their kid was the right weight,'' she said.
''And 30 per cent of the parents of obese children thought their child was the right weight.
The challenge for policy-makers would be to promote better health to parents of young children and influence household behaviour without being seen as promoting a ''nanny-state,'' Dr Hardy said.
''It's a very difficult situation, but this is happening before children enter school and we need to get the message across while also not offending parents.''
Tim Gill, an associate professor at Sydney University's Boden Institute of Obesity, Nutrition, Exercise & Eating Disorders, has researched public health messages and how people respond.
''Parents are setting up lifetime habits for children and the most effective message is modelling that comes from parents,'' Professor Gill said.
''Parents are under a lot of pressure and they are time-poor, but they don't quite understand the impact little things like sitting down and having a family meal and seeing it being cooked and going outside to kick soccer ball even for five minutes can have.''