The Greens would support an investigation into how a 'fat tax' on processed food in New Zealand's supermarkets could be imposed.

But the government has dismissed the idea, saying it would add to the burden of families in tight economic times.

Economist Gareth Morgan is calling for the move in his new book, Appetite for Destruction, which says unhealthy processed foods should be taxed at a higher rate than healthier options as a way to combat the country's obesity epidemic.

He told TVNZ's Q+A programme fast food was only one factor of the problem, but the main focus needed to be on processed foods sold in supermarkets.


"You go through a supermarket and it's virtually all now processed food, which is not a crime in its own right, obviously, but when you do the decomposition of processed food and you see how energy intensive it is and how nutrient light it is, our bodies simply can't handle it."

Health professionals should be involved in creating a system, such as traffic light colours, where a red light labels on foods attracted the highest tax, Dr Morgan said.

Kevin Hague, Green Party spokesperson on health and wellbeing, said he would be interested to read Mr Morgan's new book as it sounded similar to party policy on combating obesity.

Clear front labelling of unhealthy ingredients and a traffic-light type classification system is a "no-brainer", said Mr Hague.

But while the Greens would endorse increased taxes on unhealthy products such as alcohol, cigarettes and sugary drinks, it may be too difficult to police a wide range of food, he said.

"In a broader sense a tax on unhealthy foods is more complex and would require a more technical process. There is some dispute among academics about how exactly to design a tax to do that."

In 2011, Denmark introduced a tax on butter, meat, cheese, pizza, oil and processed food that contained more than 2.3 per cent saturated fat, but withdrew it a year later due to difficulty in implementing it.

However, the Greens would support an investigation into how to design an effective processed food tax.

"The urgency to combat obesity is so great, we should be exploring all these avenues," said Mr Hague.

"Spending on prevention of obesity should be the absolute priority for any health minister."

However the government said processed food was unlikely to be taxed anytime soon.

"Families don't need a special tax on food and the Government has no plans to introduce one," said Health Minister Tony Ryall.

"Such a tax would add to the burden of many families in tight economic times."

The Labour Party has suggested in the past GST be removed from fruit and vegetables, which Dr Morgan dismissed.