UPDATED 8.10AM The government is being urged to listen to the experts and bring in a tax on sugary drinks.

LISTEN ABOVE: Auckland University Professor Boyd Swinburn speaks to Mike Hosking

Seventy public health specialists have written to the Government asking for the tax to try and tackle New Zealand's obesity issues.

Green Party health spokesperson Kevin Hague said a tax on sugary drinks would help curb New Zealand's high obesity rates.

"We do know that putting a tax on a product reduces consumption of that product. And we know that reducing consumption of sugary drinks will reduce obesity."

He said he refuted the Government's claim that there was not enough evidence to support such a tax.

"In fact there is stronger evidence for a levy on sugary drinks than there is for any other aspect of the government's obesity packet," Mr Hague said.

But the Prime Minister says nothing's changed regarding the government's view on a sugar tax.

John Key told Mike Hosking taxing a can of coke sounds simple, but it's not.

He said you can't just say it's only sugary drinks - what about fast food, the lack of exercise and lack of public understanding.