But John Totini's employer at the time of the incident, Leader Security, still faces civil action over the uppercut that left the victim with medical bills worth an estimated $30,000.

Totini, 22, pleaded guilty in the ACT Supreme Court to causing grievous bodily harm.

The court heard Totini, a promising rugby player, was working at Northbar in December 2011 when he was told his partner had been pushed down the stairs at a nearby nightclub.

An ambulance was called and the injured woman, who had suffered dislocated shoulder, was helped into the back of the vehicle.

Totini mistakenly mistook the victim for the culprit and punched him, causing the victim to fall backwards and hit his head on the pavement.

The assault knocked the man unconscious and left him with a broken jaw, major damage to his teeth, a laceration to his tongue and a ruptured ear canal.

The 26-year-old Aranda man underwent surgery and orthodontic treatment as a result of the attack.

The court heard no offer of reparation for the medical costs had been offered or sought.

But Maurice Blackburn lawyers on Friday announced the victim would sue Leader Security.

The victim's lawyer Andrew Finlay said Totini was employed as a security guard by the Canberra firm to protect patrons, not assault them.

Mr Finlay said the assault had severely disrupted the victim's work and social life and he had to live with permanent physical and psychological complaints.

Totini was initially charged with intentionally inflicting grievous bodily harm and negligently causing grievous bodily harm by an unlawful act.

He pleaded not guilty and was committed to stand trial on a downgraded charge of recklessly inflicting grievous bodily harm.

Totini switched his plea to guilty when the charge was reduced to causing grievous bodily harm.

Chief Justice Terence Higgins on Friday said the incident showed "the folly of taking the law into your own hands".

The judge convicted Totini of the offence and ordered him to sign a one year good behaviour order.