A security guard has been hit over the head with a full bottle of alcohol during a bottle shop robbery in Canberra's south overnight.

The robbery began at about 9pm last night when a man entered the BWS Liquor store in Calwell, took two bottles of liquor from the shelf and left the store.

A security guard then approached the man and asked him to pay for the items. But the man lashed out, hitting the guard in the head with one of the full bottles of alcohol, cutting his forehead.

A manager from a nearby store came to the guard's aid and wrestled with the offender, who broke away and ran from the scene.

A subsequent police search a short time later failed to locate the offender.

He is described as Caucasian, about 180cm (5'11”) tall, with medium to solid build, with brown hair. He was wearing a dark jacket, dark-coloured Canterbury track pants with a thick white stripe down the side, and dark-coloured shoes.

The security guard was taken to The Canberra Hospital for treatment.

Police are urging anyone who may have seen this offender fleeing the Calwell shops in the direction of the loading bay after the incident (around 9pm), or may have any other information which could assist the investigation, to call Crime Stoppers on 1800 333 000, or via the website.