Young thieves are targeting Northland supermarkets and using their knowledge of their "rights" to flee without being stopped by store workers.

Police, supermarkets and the union representing supermarket workers, say it's a concern that the young thieves are clued-up on their legal rights and using that knowledge to avoid being caught.

The issue was highlighted by Northern Advocate editor Craig Cooper, who witnessed a shoplifter in Countdown Kensington in Whangarei, alerted staff, and then watched as staff were virtually powerless to stop the young thief walking out of the supermarket with his stolen item.

Shoplifters caught on the premises have not legally committed a crime until they leave the store - even if they are concealing items - and children caught stealing are aware of this loophole. They also know they will not be pursued by Countdown staff who are trained to not approach thieves outside the store.

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Whangarei police Acting Sergeant Dan Cleaver said part of the problem was the "grey area" in the law that the youths were well aware of around "reasonable force".

"Technically as soon as you apply force to somebody else it's an assault, but reasonable force is then justified under law, but it has to be reasonable force under the circumstances. So is it reasonable to apply force to and detain a child over a $2 bag of chips?" Mr Cleaver said.

"The reason police don't recommend physically restraining people is that it would be easy for that line to be crossed; particularly when dealing with a young child, the level of force doesn't need to be that great to become an assault. And if things kick off it can escalate quite quickly."

He said police had been working with supermarkets in the region for the past two years over shoplifting - including staff training and advice on store layout - and that work had led to a reduction in the number of opportunistic thefts.

"The amount of shoplifting hasn't really increased, but it's more organised and targeted and we are working with the supermarkets to reduce that also."

Countdown national loss prevention manager Bruce McKinnon said shoplifting did not create a huge loss for the company, but it took it seriously and gave all its staff training on how to deal with shoplifters.

Mr McKinnon acknowledged that staff could do little to stop a young shoplifter if they did not remain inside the store when asked to do so.

"It sounds like in the incident witnessed by your editor, the staff member reacted exactly like we expect them to and did a pretty good job in handling the situation," Mr McKinnon said.

He said CCTV footage of suspects was passed on to police to follow up and he was satisfied with the way police handled the issue.

Garry Hetherington, of First Union which represents supermarket and store workers, said shoplifting was an issue for all supermarkets and large stores in Northland and it was frustrating for union members that they could do little to stop thieves.

Mr Hetherington said he was concerned for the union members' safety as shoplifters would get more emboldened the more they got away with theft.