A Canberra man didn't plan on stealing them when he put them in his basket during his weekly grocery shop, but had a sudden urge not to scan them when he got to the self-serve checkout.

"I had never stolen from anywhere else," he said.

"But I just thought about how much the big supermarkets were ripping off farmers and under-paying staff and decided not to scan them."

Two years later, he estimates he regularly pays for two-thirds of his groceries when using self-serve checkouts.

The 22-year-old university student has opened up about why he steals in light of Coles revealing about one-third of all shoplifters caught in their Canberra stores were at self-serve checkouts.

Meanwhile ACT Policing is providing a free service to local businesses on how to protect themselves from crime, and NSW Police are teaming up with Coles to crack down on shoppers who lie about what they're purchasing to get a discount or avoid paying.

Canberra's confessed supermarket thief is an example of people "neutralising their moral concerns" by taking from companies they believe have acted unethically, which was a common self-rationalisation for stealing, according to criminologist Adrian Beck.

Professor Beck's research found self-service checkouts could be "normalising" theft in supermarkets.

The university student referred to Coles and Woolworths putting suppliers under pressure and underpaying penalty rates to low-paid workers.

"I know that two wrongs don't make a right, but if they couldn't afford so many people doing this at their self-serve checkouts they wouldn't have them," he said.

And he's likely not alone in this perspective, if ANU criminologist Emmeline Taylor's research is anything to go by.

She reviewed surveys around the world and found up to one-third of customers regularly steal when using a DIY checkout to pay for their groceries.

ACT Policing did not have data on self-serve theft, but Coles said that 315 of the 909 people caught shoplifting at stores across ACT since January had done so at self-serve checkouts.

The university student said he would never steal directly from a supermarket aisle and would not take more than his "moral compass" could handle.

"I always feel a little bad when I've stolen more than I've paid for, so I try to keep at at about a third [of stolen items]," he said.

"Some people ask me why I don't just shop at smaller supermarkets whose businesses I actually support and pay full price, but I can't afford to."

He called himself Robin Hood because "I'm stealing from the rich and giving to the poor".

A Coles spokesperson said the self-serve checkouts were a convenient choice for customers but the company was working with police across Australia to catch more thieves.

A Woolworths spokesperson said it would "continue to explore all options [for customer payments] and assess their suitability" but the supermarket had faith in Canberra customers doing the right thing. Woolworths would not provide figures of self-serve checkout thefts in the ACT.

In the ACT, the offence of theft worth up to $2000 carries a maximum penalty of $7500, six months' imprisonment or both.

ACT Policing stressed that shoplifting would not be tolerated and urged customers to notify staff or security if they see someone acting suspiciously.