Retailers such as Coles say they're sick of being targeted by opportunists who lie about what they're purchasing to get a discount or who try to avoid paying at all.

"No matter how small you think it is, even if it's the avocado and you're saving $2, it's still shoplifting," detective superintendent Murray Chapman told reporters outside the chain's supermarket in Zetland, in inner-Sydney, on Thursday.

Up to one-third of customers regularly steal when using a DIY checkout to pay for their groceries, according to a recent review of surveys around the world by Australian National University criminologist Emmeline Taylor.

Dr Taylor last month told Fairfax Media one of the biggest issues with self-scan was that otherwise honest customers seemed to regularly use it to underpay for groceries.

These shoppers often got their start in stealing by accidentally not scanning an item or scanning a grocery item for a cheaper item.

"But upon experiencing how easy it was they continued to steal regularly," Dr Taylor said.

Criminology professor Adrian Beck from the University of Leicester in Britain earlier this year warned self-service checkouts could be "normalising" theft in supermarkets.

"People are very good at neutralising their moral concerns when thinking about stealing things ... and people can end up feeling they have a right to get their share of the corporate profits," Professor Beck said.

Checkpoint Systems' Mark Gentle, who works with major supermarket chains to protect their stores from losses or theft, recently said theft was on the increase in Australia.

Mr Gentle said people would be surprised to know how many businesses and even cash-strapped pensioners use the grey-market to buy expensive groceries such as meat at considerable discounts.

Last month, a spokesman for Coles said the "vast majority" of its customers did the right thing and Woolworths said it had "strong measures" in place to limit stock loss.

Catie Low with AAP