The birth of Mandy Strickland's first son was one of the happiest moments of her life, but a throwaway remark from a midwife about her weight tainted it.

The Bendigo woman had always struggled with her weight before she gave birth to a 4.54 10 pound two ounce baby, which prompted a nurse to link his larger size to her being overweight.

"I felt horrible," Ms Strickland told AAP.

"I thought 'I'm not even a mum yet and I'm a bad mother. My baby is big because of me'.

"I was alone at that point because it was an emergency caesarean and I told my husband later on because I didn't want to say anything."

Another midwife later clarified her son's initial size was genetic, with bigger babies tending to come from her husband's side of the family.

Seven years on, the now 45-year-old acknowledges the upsetting comment was a turning point.

She dropped more than 30 kilograms from 104kg - and has kept off the weight - after seeing a doctor who explained her problems were also related to genetics.

"When I had my son I was like 'what if he will be genetically bigger' and 'I don't want him to be bullied in school like I was - how is he going to be able to deal with it if I can't deal with it myself?'," she said.

Her story mirrors the findings of a new survey on the attitudes and experiences of overweight and obese Australians, released on Friday to coincide with World Obesity Day.

The independent survey, which polled 1000 Australians with a body mass index (BMI) of less than 24.9 and 1000 others with a BMI greater than 25, found one in five of the latter believe, or are unsure, the former deserve help and support.

Some 22 per cent suggest those classed as overweight or obese had "done it to themselves", while others agree they are lazy (23 per cent) or a burden on the health system (15 per cent).

However, one in four respondents are empathetic and 40 per cent understand a person's weight to be a medical issue rather than simply owing to a poor diet and lack of exercise.

Research shows up to 70 per cent of obesity cases can be linked to genetics, family history and ethnicity, and Monash University metabolic neuroscientist and obesity researcher Brian Oldfield says the public is "largely uninformed" on how it dictates weight gain and loss.

"If we understood better the dramatic, irrepressible physiological changes that occur in our bodies after weight loss that tend to draw us back to our original weight, we would be less inclined to judge those who find this task difficult," Professor Oldfield said.

The poll also highlights only 43 per cent of overweight and obese respondents have talked to a GP about weight management and 53 per cent judged themselves for seeking surgical or medical interventions.

In response, Danish pharmaceutical giant Novo Nordisk, who funded the independent survey, has launched a campaign called "The Truth About Weight" and is urging those concerned about their size to speak to a GP or healthcare professional.