National MP Mark Mitchell has broken ranks with his leaders after Judith Collins and Gerry Brownlee both laid the blame for obesity on personal responsibility.

Mitchell said this morning that the issue was "more complex" than Collins has claimed.

"Part of it's a personal choice," he told Newstalk ZB's Mike Hosking, "but it's a lot more complex than that".

"Some obesity can be dealt with through exercise and diet, there's no doubt about that.

"But some obesity is related to medical conditions, even psychological conditions that need treating, so it's a more complex issue".

Earlier, National Party deputy leader and MP for Ilam Gerry Brownlee said there is no one else but himself to take responsibility for his large size.

"It's mine," the politician told ZB's Hosking this morning.

It comes as National leader Judith Collins doubled down on her obesity stance, describing it as a weakness and saying that people should not "blame systems for personal choices".

She said people who were obese needed to take some personal responsibility.

Brownlee, National's deputy leader, backed her comments, saying it was not an easy topic to broach.

He said he was against any state-mandated control on obesity, supporting Collins' remarks over people taking charge of their large weight.

"It is an issue that's not easy to deal with. She was saying take personal responsibility.

"To say that there should be some state-mandated action enforcing people to be a particular size, I think that's a step too far," he said.

This morning the National leader continued addressing the country's obesity problem saying it was a matter of personal responsibility.

She told MediaWorks that it was a case of eating healthy fresh food and exercising.

"I've seen it in my own family," she said.

"People have taken charge of their food and strangely enough they've lost weight.

"Any decent GP will say it's not that complicated."

She added fresh fruit and vegetables were a significantly cheaper option that processed foods, adding a meal of home cooked chicken and veges was a great value and healthy option for dinner.

Collins said she did not look at people and think "they're really fat".

It was a matter of speaking truthfully about the subject, she said.

"You can take charge of your life.

"It doesn't take much to get frozen veges in the freezer. It's not that hard."

Earlier when told that some had called her comments heartless, Collins said: "Do you know what is heartless? Is actually thinking someone else can cure these issues.

"We can all take personal responsibility and we all have to own up to our little weaknesses on these matters.

"Do not blame systems for personal choices."

Statistics from the Ministry of Health show that one in three New Zealanders over the age of 15 are obese.

Those living in areas of socio-economic deprivation are also more likely to be obese than those living in the least deprived areas.

In addition, the statistics show the prevalence of obesity among adults differs by ethnicity, with 67 per cent of Pacific, 48 per cent of Māori, 29 per cent of European/other and 14 per cent of Asian adults obese.

About one in nine children aged two to 14 are obese.

Earlier Labour leader Jacinda Ardern was asked about obesity.

"I think on an issue like this, people are, we are all, products of our environment. You can't deny that and so we do have to look at all the multiple factors that contribute to obesity issues in New Zealand.

"I think if you are so simplistic simply to call it an issue of personal responsibility, then it's never going to be an issue that we collectively resolve."

Ardern thought it demonstrated that "under National we won't see any progress on the issue".

"If it's just a view that they've got no role to play and that there's no difference that government can make on these issues, then it does tell you that on one of the most significant health issues we have you'll see nothing from the opposition on it."

New Zealand First leader Winston Peters was asked about the issue as well.

"There's a tsunami of obesity problems coming down our track, it's a critical matter and our health system faces a nightmare unless we get going right here and right now to do something about it."

Asked if it was a matter of personal responsibility or external factors, he said: "It's a combination - frequently it's external factors, frequently it's some people sadly [have] got two or three jobs - their chances of actually stopping to ... follow good dining practices is not affordable. They are flat out with takeaway meals and what have you.

"We can all condemn them and say what we like but the reality is, they'll have sadly truncated lives and many illnesses which are avoidable and I'd like to think that this country has a seriously practical dialogue about it rather than just condemning people."

Looking forward to after the election, Collins today said the first thing she would do if elected to power on Saturday would be a quiet sit down with a hot drink.

"I'll probably sit down and have a cup of tea and bring back a lot of personal responsibility," she laughed.

Brownlee this morning told Hosking that a wealth tax would be a high priority for any Green-Labour coalition.

When it came to negotiating with the left, all taxes were back on the table.

"The only thing they've got is tax so why stop at getting an extra $500 million in, so it's a live issue," he said.

Brownlee said the Labour Party was now trying to deflect from the Green Party stance.

- RNZ and NZ Herald