Magistrate Beth Campbell told Mane Tupuola, 22, he had been "extraordinarily fortunate" that the victim had not suffered permanent injuries as it would have destroyed both their futures.

Tupuola, of Giralang, pleaded guilty in the ACT Magistrates Court to assault occasioning actual bodily harm.

CCTV footage of the assault shows an intoxicated Tupuola become aggressive after security refused him entry to the Academy nightclub in Civic at 12.40am in August 17.

He was pushed away by friends, but then argued with a bystander leaving the club.

The offender then punched the man, the force of the blow knocking him unconscious and causing him to fall backwards and hit his head on the ground. The victim was hospitalised, and released the same morning.

The court heard he suffered a cut to the back of his head, but no evidence had been provided to show if he suffered permanent damage.

Tupuola fled to NSW immediately after the attack, but was identified via the footage.

Defence lawyer Paul Edmonds said his client had become frustrated at being denied entry and lashed out at an innocent bystander.

The court heard the offender had moved to Australia from New Zealand for a fresh start away from anti-social peers.

He had been directionless at first, but now worked full-time as a stone mason and had been working towards gaining his trade qualification.

Mr Edmonds urged the court to be cautious not to adopt a knee-jerk reaction after a number of high-profile deaths from one-punch attacks in NSW.

The defence lawyer said his client had a criminal history clear of violence and deserved to be given a chance.

He said his client realised that it had been "by the grace of God he's not facing a more serious charges".

But prosecutor Michael Reardon argued Tupuola had shown limited insight and remorse into his offending behaviour and had attempted to shift blame onto security and the victim.

The prosecutor described it as a "wanton, unprovoked attack on an unsuspecting victim who was a complete stranger to the offender at the time".

"It was completely devoid of justification," the prosecutor said.

"His remorse is limited to self-pity."

Mr Reardon said this "attitude of entitlement" revealed the offender had not accepted full responsibility for his actions.

Ms Campbell said Tupuola - who she described as a "big unit" - had significant moral culpability for the "cowardly" attack.

"I flinched when you struck him and he fell to the ground," Ms Campbell said.

The magistrate said it had become well-known that a single punch could cause catastrophic injury or even death.

"You are in the extraordinarily fortunate position not to have destroyed both your future and the victims," she said.

Ms Campbell sentenced Tupuola to eight months jail, to be fully suspended upon entering an 18-month good behaviour order, with 200 hours community service.