New moms fret about gaining "baby weight" but dads are the ones piling on the pounds to a dangerous level, an Australian study has found.

Time-poor, exhausted and much fatter than they realize, fathers with young families are a ticking health time bomb, research by health business Bupa shows.

Dads with kids at home are the most at risk. Three-quarters are overweight or obese, while a third believe they are slimmer than they are.

"Women are much more realistic in terms of their weight," Bupa chief medical officer Dr. Paul Bates said.

"The data showed seven in 10 men were actually overweight of those surveyed, but only three in 10 rate themselves as being overweight. With women, six in 10 were overweight but five in 10 rated themselves as being overweight."

The study found 72 percent of fathers under 40 with young families are overweight or obese while 79 percent of dads aged over 40 living with kids also tip the scales to an overweight rating.

Bates said it was clear dads were not making the link between weight and health.

"From a family point of view, it you are overweight, your kids' chance of being overweight is much also greater," he said.

"Parents need to set a good example by exercising and eating well. Being overweight gives you a much greater risk of developing diabetes, high blood pressure and arthritis."

Bates said the alarming difference between the men's perception of how fat they are and reality might be a result of society getting heavier.

"Australia is unfortunately one of the more overweight countries in the world," he said. "We measure by an absolute standard, the BMI [body mass index], but the way you judge yourself is how heavy you are compared to the people around you."

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