Ms Turnbull said most people probably didn't know what a healthy weight was.
"Unless they've gone to the GP, and they've said, 'This is the weight you should be', I don't think people actually know.
"A lot of people focus, particularly at this time of year, on changing what they eat based on just their weight. While that is really important ... the quality of the food that you're eating affects your health and well-being and how well your body works."
Ms Turnbull said some who perceived themselves to be a healthy weight were actually undernourished.
People should also be wary of alcorexia - a diet reportedly popular with some celebrities and young women, which involved reducing calorie consumption during the week to compensate for calories consumed in weekend drinking binges.
The diet was seen as a way of maintaining a superficially healthy body image without sacrificing a party lifestyle.
Ms Turnbull said social media and pervasive gender stereotypes continued to make women and girls far more likely than men to be self-conscious about weight.
She said girls as young as 6 were visiting private practices voicing concerns about what they looked like.
People should remember that many images flooding platforms such as Instagram were edited or filtered to make people seem more attractive or skinny.
The Southern Cross survey also found most New Zealanders were ignorant of what good fats were, and underestimated their sugar intake.
Ms Turnbull said there was widespread confusion about good and bad fats.
She recommended fats from extra virgin olive oil, avocados, nuts and seeds.
"Fats are really important to your health and well-being but it's not fats in doughnuts and cream buns and pavlova."
New Zealand Nutrition Foundation dietician Sarah Hanrahan said people on diets should use the holiday season as a chance to make "some simple long-term lifestyle changes".
Southern Cross Health Society chief executive Peter Tynan said it was known most people struggled to keep the kilos at bay, "which becomes a problem when the excess of Christmas is over".
"The risk of disease increases as BMI [body-mass index] increases, so while some might pass off Christmas weight gain as being festively plump, if you can't lose the weight, the long-term consequences are pretty dire," he said.
The survey asked people to note their perceived BMI. Hamilton and Tauranga reported the highest average.
According to the Ministry of Health, adult obesity rates were increasing, with 31 per cent of all adults obese and more than 5 per cent of New Zealanders morbidly obese.
Obesity rates for children in the most deprived neighbourhoods were five times those of children living in the least deprived neighbourhoods, the ministry said. Adults in the poorest neighbourhoods were 1.7 times more likely than those in the least deprived areas to be obese.
Weighty issue
28 per cent of all respondents described themselves as "quite a bit overweight".
40 per cent described themselves as "overweight".
21 per cent of Kiwis said their doctor had raised their weight as a concern, with this rising to a quarter of Cantabrians.
27 per cent of women believed their weight was "about right", compared with 36 per cent of men.
