Sydney - Fathers who play less of a role in child rearing are more likely to have overweight or obese offspring, Australian researchers said.

A study of almost 5,000 youngsters revealed that fathers who do not set clear limits for their kids are more likely to have heavier children, according to the Australian Associated Press.

Dads who did lay down boundaries generally had children with a lower body mass index, said the study by the Murdoch Children's Research Institute and The Royal Children's Hospital Melbourne.

Hospital specialist Melissa Wake said the study was the first to suggest that men could help prevent early childhood obesity.

The study found that a mother's parenting style had little impact on whether a child was overweight or obese.

"Mothers are often blamed for their children's obesity, but this study suggests that for more effective prevention perhaps we should focus on the whole family," Wake told the AAP.

The study found that 40 percent of mothers and more than 60 percent of fathers polled were themselves overweight.

The research, to be presented at a pediatrics conference in Toronto this week, compared the BMIs of 4 and 5-year-olds with their how their parents behaved towards them.

Childhood obesity is growing in Australia, with more than 20 percent of preschool children either overweight or obese.