UNSCRUPULOUS shoppers are stealing millions of dollars of goods from supermarkets by scanning expensive personal products as "carrots and onions" at self-serve checkouts.

The situation has become so critical that Woolworths has installed a locking and lights system to alert staff when "suspicious" quantities of cheap items are being bought.

A recent data check by the supermarket of its self-serve checkouts revealed more than 1000 transactions involving more than three bags of carrots in one week. This included a lone shopper scanning 18 bags of carrots.

Its rival Coles said its self-serve checkouts were also fitted with hi-tech anti-theft systems - but that the chain had not experienced a rise in thefts.

Woolworths has installed more than 3000 self-service checkouts in 500 stores, while Coles has about 3000 in 545 stores.

Both supermarket giants guard loss figures, but the Australian Retailers Association estimates retailers lose about 3 per cent of sales each year, equating to more than $100 million each.

An industry source said the self-serve checkouts had spawned a new breed of thief - one who ordinarily would not think of stealing but justified giving themselves a discount.

"There is a high frequency of otherwise upstanding citizens who don't see any harm in challenging the system and when they are caught, they are very, very embarrassed and remorseful," the source said. "But theft is theft."

A Woolworths spokeswoman said the supermarket began trialling data checks from its self-serve checkouts earlier this year.

The spokeswoman said carrots and onions were popular cheap items that opportunistic shoppers scanned in place of more expensive items.

She said the supermarket had upgraded its security and CCTV systems, while re-programming the checkouts to lock the system when a large quantity of a cheap item was being scanned.

The new security measures had been rolled out in the past few weeks and had already resulted in a decline in thefts, the spokeswoman said.

She said the measures had led to a drop in detected theft at self-serve checkouts to less than 200 cases a week and estimated it would save Woolworths more than $1 million a year.

The latest ARA figures show retailers lost $7.5 billion to theft in 2011, or about 3 per cent of retail sales.

This is up from $6.8 billion in 2010 and $5 billion in 2009.

linda.silmalis@news.com.au