A man who bashed a fellow drinker at the Belconnen Labor Club last year told police he struck the victim first, but did so in self-defence.

Leonard Anthony Subritzky, 35, of Belconnen, was playing pool and drinking with the victim when he suddenly punched him in the face twice, knocking him to the ground.

He pleaded guilty to one charge of common assault in the ACT Magistrates Court on Monday.

The court heard Subritzky was identified by a security guard at the club.

According to a statement of facts tendered in court, the victim and his partner were at the club on April 30, where they watched football, played poker machines and had drinks.

They were playing pool when they first met Subritzky and his girlfriend and the two couples played several games together.

As the evening wore on, the victim noticed that Subritzky appeared drunk and seemed to suffer mood swings, appearing friendly at times and agitated at others. Just after midnight, the victim and his partner went to the bar and bought a round for the two couples.

The victim had just set the drinks down on a nearby bar table when Subritzky walked up to him and punched him in the nose and cheek.

The court heard Subritzky, who was more than 30 centimetres taller than his victim, struck the man with such force that he knocked the bar table over and sent the man sprawling on the ground.

Subritzky then picked the victim up and put him in a headlock, but security staff intervened and forced him to leave the club.

The court heard the incident was captured on CCTV footage.

Subritzky was caught a week later when he returned to the club and a security guard identified him.

He told police he was acting in self-defence.

Subritzky said he felt intimidated because the victim appeared agitated and had threatened to smash him in the face with a glass.

He will be sentenced at a later date.