Now he will spend nine months behind bars for the attack which left his victim, aged in his 20s, unconscious on a Civic footpath with a shattered jaw in the early hours of January 1.

On Friday Sharma, 20, was sentenced in the ACT Supreme Court to two years and three months imprisonment, to be suspended from April next year.

Justice Michael Elkaim said the court faced a difficult task in sentencing Sharma – a young offender with an unblemished background who had committed "a single act of immense stupidity".

Sharma, 20, handed himself in after ACT Policing released CCTV footage of the attack a week after it took place.

He originally pleaded not guilty to recklessly inflicting grievous bodily harm but later switched his plea to guilty.

Footage of the incident showed the male victim had an argument with a man outside the East Row Supa 24 convenience store as Sharma stood nearby about 3am.

Sharma then clenched his fists, adopted a "fighting stance" and threw a single punch at the victim's jaw as he looked the other way, causing the man to fall and hit his head on the ground.

The two men can then be seen to walk away without helping the victim.

Police were called and found the man unconscious on the ground soon after.

Doctors had to insert a titanium plate and four screws into his jaw to fix the damage and it was wired shut for two weeks.

Sharma went to City police station in Civic several hours after the footage became public, admitting he was the person in the CCTV footage and that he threw the punch.

Justice Elkaim said Sharma had been highly intoxicated and "no doubt fuelled with the bravado generated by his alcohol consumption" when he struck the man's jaw.

The court heard the victim had endured severe pain due to his injuries and the attack had a marked psychological impact as he continued to battle depression, bouts of anger and a curbed social life.

Justice Elkaim said a single punch was less severe than a sustained attack involving a series of blows, and noted the victim's injuries were sustained in the initial punch and not from hitting the ground.

"However a single punch, especially when made to a victim's head so that he is rendered unconscious and falls to the ground, has the potential to result in the most severe of consequences."

He said the footage showed the callousness of the attack, which involved an element of reckless behaviour given he had consumed a large amount of alcohol.

"Although it was not premeditated there is a clear indication of some forethought as the offender adopted a stance preparatory to the striking."

The court heard Sharma had the support of a devoted family, had made efforts to reduce his alcohol consumption and referees had described the attack as out of character.

He had also written a letter to his victim, expressing sorrow for his actions and hoping the man would one day forgive him, although Justice Elkaim said he believed true contrition would have seen the letter written earlier.

Justice Elkaim said any sentence needed to involve "a very high degree of deterrence" and reflect society's abhorrence for such attacks.

"The courts must emphasise to young people that the consumption of large amounts of alcohol, or drugs, that places them in situations where they act with reckless indifference towards other persons and cause very severe injuries is entirely inappropriate.

"As has tragically been seen around Australia, the tragedy can include the death of the victim."

He noted Sharma had taken steps toward rehabilitation but said a term of imprisonment was inevitable.

Sharma acknowledged his supporters in the courtroom, who appeared shocked by the judge's decision, but did not react as police led him away in handcuffs.