The environmental "holy grail" that is the reusable shopping bag is facing scrutiny after claims they are only better than plastic bags on paper.

A new study shows reusable cotton bags need to be used 173 times before they're better than single-use plastic bags for the environment.

Victoria University's environmental science lecturer Dr Lynda Petherick told Mike Hosking reusable bags are harsh on the environment to produce.

"I think sometimes people might just have a bit of a knee-jerk reaction, you see something that looks better on paper but isn't necessarily better when you look at the entire lifespan."

"Things like single-use plastic is really quite cheap in terms of energy and environment to produce but obviously has pretty significant and obvious negative impacts in terms of disposal."

"Cotton on the other hand, is quite expensive in terms of the environment and what it costs to produce it, if you're looking at water consumption, for example, and the amount of fertilisers and pesticides that are applied to cotton when it's being grown."

She said people it's not realistic to think that people will use their bags enough the make them environmentally friendly.

"that sort of equates to using the cotton bag once a week for three and half years."

This follows Finance Minister Grant Robertson admitting, over the weekend, that he has to buy a new reusable bag each time he goes to the supermarket.

LISTEN ABOVE AS LYNDA PETHERICK SPEAKS WITH MIKE HOSKING