A Chief Economist says there are substantial problems in the research supporting a sugar tax.

It comes after The New Zealand Initiative were released documents under the Official Information Act revealing the Ministry of Health's scepticism towards a sugar tax.

They revealed advice to then Minister Coleman that there was insufficient evidence a sugar tax would reduce obesity.

The Initiative's Chief Economist Dr Eric Crampton told Mike Hosking public health experts presented a well-supported campaign in favour of a sugar tax, but he thinks the documents suggest something different.

Crampton agrees with the Ministry and says there are problems with the way the data is measured. 

"There are substantial problems in the way that they've the estimated the attack on consumption. Those problems would be obvious to economists but not to public health practitioners.

"If an excise tax comes in that increases the price of soda, maybe you're not spending as much on it because you've shifted from Coke or Pepsi down to homebrand."

Crampton says you could potentially be consuming more sugar, while reporting you're consuming less.