Research into how teenage girls view their weight has shown many of those overweight believe their size is normal.
The Auckland University of Technology study surveyed 945 girls aged between 11 and 15 about perceptions of their weight.
Of those surveyed, 142 girls had a body fat percentage which classified them as being overweight.
However, almost half of these overweight girls incorrectly considered themselves to be of normal weight, or even underweight.
Nutritionists say that perception is a great public health risk and will make it difficult for overweight girls to engage in healthier lifestyles.
Conversely, the study also showed about half of all the girls surveyed were actively trying to lose weight - meaning normal and underweight girls considered themselves overweight.
The study warns that underweight girls who perceive themselves as being overweight are at risk of developing eating disorders.
The Obesity Action Coalition says it is not surprised many obese girls think their weight is fine.
Spokesperson Bronwyn King says it is evident in everyday life that obesity is becoming accepted, and it is going to take a shift in society's attitude to change incorrect perceptions of healthy weight.