A man will spend at least five years in jail over the one-punch manslaughter of a 19-year-old who was trying to protect a friend during a Melbourne pub brawl.

Patrick Cronin died after being punched in the head while trying to pull his friend away from the fight outside The Windy Mile hotel, at Diamond Creek in Melbourne's north-east, in April last year.

Andrew William Lee, 34, had pleaded guilty to manslaughter after a deal was struck with prosecutors to have a murder charge downgraded.

He was today sentenced to eight years' jail.

He will be eligible for parole in five years.

In sentencing, Supreme Court Justice Lex Lasry described the punch as "spontaneous".

He said Lee had shown genuine remorse and had good prospects for rehabilitation.

'A terrible mistake'

Justice Lasry said Lee had been trained in the martial art of Muay Thai and should have known his skills should only be used in self defence.

"Your actions were spontaneous, which occurred over a few seconds," Justice Lasry told Lee.

Andrew Lee was genuinely remorseful, the court found. ( AAP: Julian Smith, file photo )

"You made a terrible mistake. You are remorseful for what you have done.

"Prior to this matter you are a person of good character."

Outside court, Mr Cronin's father Matt Cronin said Lee's "life sentence" would be having to live with what he had done.

"Pat ultimately died doing what we taught him to do, which is look after his mates," he said.

"He had a choice that night, he could have thrown a punch, he could have added to the violence of the situation but instead he chose to help his mate.

"Andrew Lee, he had exactly the same choice, but instead he chose to throw a coward's punch, and that punch killed Pat.

Matt and Robyn Cronin were critical of Victoria's one-punch laws. ( ABC News: James Oaten )

"The sentence that's been given today was never going to be enough. We resigned ourselves to that fact a long time ago."

Earlier, he told ABC Radio Melbourne that Victoria's coward punch laws, which impose minimum 10-year jail terms, had failed his son.

Lee was not charged under those laws.

"Because it's one-punch they said we can't prove intent, it needs to be a sustained attack [for those laws to apply]," Matt Cronin said.

"The lawmakers need to go back to the drawing board."

'One punch' law review

Victorian Attorney-General Martin Pakula described the death as a tragedy and said while it was not appropriate for him to comment on individual cases, the Government would work with the state's Director of Public Prosecutions (DPP) to review laws.

"When in Opposition we voted for the former Liberal Government's one-punch laws," he said.

"We've now had two cases where the DPP has determined that the statutory minimum wasn't applicable given the way those laws were drafted.

"We will work with the DPP to establish whether that law should be amended to broaden the circumstances in which prosecutors can seek to apply the statutory minimum."

Victim was trying to calm brawl

On the day of his death, Mr Cronin had played his first senior football match for his local club, Lower Plenty, alongside his older brother.

Earlier hearings were told Mr Cronin had taken no aggressive part in the brawl, and had only been trying to help his friend, Anthony Hopkins, who was also struck in the head by Lee.

Patrick Cronin was attacked at The Windy Mile. ( ABC News )

The court heard Lee had approached Mr Cronin forcefully, before hitting him with a powerful punch that sent him stumbling sideways.

"It must have been obvious to you that Patrick Cronin's actions were to defuse the situation," Justice Lasry told Lee.

Mr Cronin continued helping his friend and it was not until about an hour later that he started feeling unwell.

His condition quickly deteriorated and he suffered bleeding on his brain.

He died in hospital days later.

Foundation pushes for change

Robyn Cronin told ABC Radio Melbourne the loss of her son had "broken us".

Patrick Cronin had played his first senior football game on the day he was killed. ( Supplied )

"We've lived without Pat for nearly 19 months now.

"It still just doesn't feel real, in so many ways, that he's gone.

"I still can't fathom how we got here and how this happened."

The family has set up the Pat Cronin Foundation to raise awareness about one-punch attacks and to lobby for legislative change.

"There's really only one victim and that's Pat," Matt Cronin said.

"Pat hasn't had a voice since April 16. We're his voice."

"We just feel that the way the system works is we don't really have anyone in our corner."