The man who killed 18-year-old water polo player Cole Miller in Brisbane's Fortitude Valley in a random one-punch attack has been sentenced to seven years in prison.

Armstrong Renata, 23, pleaded guilty to unlawful striking causing death in the Supreme Court in Brisbane.

Cole Miller died in the early hours of January 3, 2016, after a one-punch attack. ( Supplied )

Renata punched Mr Miller in the back of the head while on a night out in the party precinct in January 2016, causing the teenager to fall and hit his head on the pavement.

Mr Miller, a former Brisbane State High School student, was walking through a mall with a friend to catch a taxi home when he was set upon.

The promising water polo player suffered severe head injuries and massive brain trauma and his life support was turned off the following day.

Thousands of people attended vigils and a rally in King George Square to show support for the Miller family shortly after and called for an end to violent behaviour.

Renata's co-accused Daniel Maxwell, who started the fight after the pair had been kicked out of a venue, was given an 18-month sentence in August for assault and was deported back to New Zealand.

A court sketch of Armstrong Renata during the sentencing. ( ABC News: Milly Capel )

The families of both Mr Miller and Renata were in court for the sentencing.

Renata has to serve 80 per cent of his sentence, and will eligible for parole in about four years, given time already served.

Justice Helen Bowskill said she took into account his early plea of guilty and the six months Renata spent in solitary confinement in a maximum-security prison.

Renata is the second person to be sentenced under Queensland's one-punch laws.

'Miller would not have seen the attack coming'

Prosecutor David Meredith said Mr Miller and his friend Nick Pace had no contact with Renata and Maxwell until the fatal event.

Unlawful striking charge explained "Unlawful striking causing death" is defined as hitting another person on the head or neck and subsequently causing the death of that person

"Unlawful striking causing death" is defined as hitting another person on the head or neck and subsequently causing the death of that person It was added as a new offence in the Queensland Criminal Code in 2014 as part of a suite of measures introduced in the Safe Night Out Legislation Amendment Bill

It was added as a new offence in the Queensland Criminal Code in 2014 as part of a suite of measures introduced in the Safe Night Out Legislation Amendment Bill The charge "fills the gap" between manslaughter and an assault that results in the death of a person

The charge "fills the gap" between manslaughter and an assault that results in the death of a person The Queensland Government's Safe Night Out laws are aimed at reducing alcohol and drug-related violence in the state's nightlife Source: Quinn & Scattini Lawyers/Queensland Government

The court heard Maxwell was drunk and had picked three fights with people, before the two groups crossed paths in the Chinatown mall.

Maxwell challenged Mr Miller to a fight and punched him, aiming for his jaw, but the blow landed on his chest.

Mr Miller did not respond and indicated he did not want to fight.

Maxwell turned to Mr Pace and punched him in the chest before Mr Pace and Mr Miller backed away retreating.

Renata then attacked Mr Miller when he was facing away from him and had his hands at his side.

"Renata moved around behind Cole Miller and then punched him in the side of the head," Mr Meredith said.

"Miller would not have seen the attack coming.

"The blow was of such force … it, as well as contact with ground … [meant] injury was unsurvivable.

"Miller had done nothing to Renata."

Armstrong Renata pleaded guilty to unlawful striking causing the death of Cole Miller. ( Facebook )

Mr Miller fell to the ground, seemingly knocked out, and Mr Pace tried to revive him and called an ambulance.

Renata and Maxwell went into a car park, waited for their group, and then left, while Mr Miller was on the ground.

"You left as though nothing had occurred," Mr Meredith said.

Defence barrister Angus Edwards told the court his client had genuine remorse over Mr Miller's death and had "perceived, wrongly … that Mr Miller was going to hit his friend so he struck first".

Cole Miller was a member of the Brisbane Barracudas Water Polo team. ( Facebook: Brisbane Barracudas Water Polo )

'Cole was a kid with a dream'

Mr Miller's father Steve Miller fought back tears as he read his victim impact statement about his family's "utter turmoil".

"His life was violently cut short … he was ready to embark on university," he said.

"Cole was a kid with a dream … his life was just beginning."

Mr Miller recounted the unexpected 4:00am phone call and rushing to the intensive care unit.

"It was a nightmare no-one should have to endure. It will never end for me or my family," he said.

Mr Miller spoke of his son's "tragic and senseless" death and said it left "a massive void" for his three siblings.

Mourners lay flowers at a vigil for Cole Miller in Chinatown, Brisbane. ( ABC News: David Lewis )

A statement read on behalf of the victim's mother, Mary-Leigh Miller, said life as she knew it came to an end when her "beautiful, quiet, gentle" youngest child was killed.

"I am feeling wounded, isolated … with no idea how to heal myself," she said.

"[I'm] not sure I can function as a normal human being again."

'I owe his family a debt that can never be paid': Renata

Renata read a letter of apology to the court and said he did not think he could ever forgive himself.

"Words can't begin to describe how deeply sorry I am," he said.

"I can't imagine the pain and trauma I have caused this family … and never in a million years would.

Cole Miller was the youngest of four siblings. ( Supplied )

"I owe his family a debt that can never be paid for.

"We had no right to do what we did and I'm sorry."

Outside court, Mr Miller's father said the problem of alcohol-fuelled violence needed to be addressed.

"Until there is a bigger deterrent, I think we're not going to solve this problem. We have a social problem with violence. And there's violence, and everyone is affected by it," he said.

"It's not how I want him to be remembered. He will be remembered as he was. Very much the best of all our family, we lost our best one."