Supreme Court Justice Elizabeth Hollingworth said the victim, David Cassai, did nothing to provoke the "gratuitous attack" by the killer, Dylan Closter, 20.

Justice Hollingworth said that in the space of less than a minute multiple punches were thrown, one man was dead and five others injured in the violent, frightening attack in the middle of the night on a busy street.

"What happened that night was entirely your making," the judge said in sentencing Coster to nine years and three months with a non-parole period of six years.

Justice Hollingworth said Closter's immature, alcohol-fuelled attack had tragically taken a much-loved and respected young man's life.

Family and friends of Mr Cassai were in court for the sentencing, wearing white T-shirts which read: "Stop. One punch can kill."

Closter had pleaded guilty to one count of manslaughter and one count of affray after he had been the one who initiated the attack on Mr Cassai.

Mr Cassai, 22, of Templestowe, and his friends had had a night out at the Portsea Hotel before taking a bus back towards his family's holiday house in Rosebud in the early hours of December 31, 2012.

But the bus only went as far as Rye, where Mr Cassai and his friends decided to walk the rest of the way to Rosebud.

As they were walking the group encountered Closter and his friends, Thomas McCluskey-Sharp, 26, and Tyrone Russell, 22.

Closter, an apprentice plumber, angrily demanded that a friend of Mr Cassai's put down a street sign he was carrying, before deliberately shoving Mr Cassai with his shoulder.

Closter then held his hands up like a boxer and began throwing punches at Mr Cassai, who was trying to back away and avoid the confrontation.

McCluskey-Sharp and Russell joined the fight, with one calling out, "Welcome to Rye, c----."

Mr Cassai fell over on the road after one punch connected, before getting up and being chased by Closter who hit him in the left temple with one powerful roundhouse punch.

Mr Cassai fell straight backwards after the punch, hitting his head on the pavement and fracturing his skull.

He died of his injuries at The Alfred hospital about 4pm on December 31.

As Closter and his friends ran off, one of them said, "That's what we came here for".

Closter had been due to stand trial on August 8 but pleaded guilty that morning to manslaughter and affray.

Russell pleaded guilty to recklessly causing serious injury, causing serious injury, and affray, and was jailed for 15 months with a non-parole period of eight months.

Details of both men's cases were suppressed until Wednesday when McCluskey-Sharp indicated he would be pleading guilty to affray and intentionally causing injury.

Amazing strength from David Cassai's articulate mum, Caterina, outside court. @STOPonepunch movement is his legacy. pic.twitter.com/1C6uFYX15c — Kate Jones (@Kate_Jones7) October 2, 2014

Justice Hollingworth said it was true Closter had not used a weapon but the number of punches he had thrown at Mr Cassai and others in such a short space of time, while they were trying to get away from him, "demonstrates the ferocity" of his attack.

"This was very much a one-sided attack. You started the affray and remained the principal aggressor throughout," the judge told Coster .

"There is a clear need to deter others from engaging in random, unprovoked, late-night street violence."

Outside court, Mr Cassai's mother Caterina Politi said she did not think the family would ever get justice.

"Six years minimum, it'll never be enough. It doesn't send a message of deterrence or punishment for a man who took my son's life," Ms Politi said.

"He wasn't a threat to Dylan Closter. David's friends weren't a threat to anyone that night. They had just been out to celebrate a birthday and they just wanted to get home.

"They were ambushed and they had no chance. David's friends couldn't even help him... They have to live with that as well, the pain of witnessing what happened and losing their friend... We'll never recover from this."

Ms Politi said the message "Stop. One punch can kill" on her T-shirt might help stop similar tragedies from happening.

"Hopefully no one will ever die from a coward punch," she said.

Ms Politi said her son was a star to so many people.

"Everyone thinks their children are fantastic but David made a difference to so many people."

Ms Politi later said the entire community needed to address the "cancer" of aggression on Victoria's streets.

"The judges can't do it on their own, the laws can't do it on their own, the police can't do it on their own - we all have to work together," she told radio station 3AW on Thursday afternoon.

"David would get drunk but he'd never go out and hurt anyone. What is it that triggers this anger?"

Ms Politi said while the focus recently had been on the influence of terrorism, she believes Australians "are doing a pretty good job of destroying each other... We don't need other influences."

Since David's death, Ms Politi has campaigned for politicians to introduce tougher one-punch laws and has been a driving force behind Victoria's "Stop. One punch can kill"  campaign.

On Thursday she said she hoped a new minimum of 10-years prison terms for one-punch deaths, soon to be introduced, would help bring a significant change in community attitudes.