The man who killed 18-year-old water polo player Cole Miller in Brisbane's Fortitude Valley in a random one-punch attack has had his sentence increased to nine years and six months after a Court of Appeal ruling.

Key points: Armstrong Renata admitted punching Cole Miller in the back of the head in a random attack in January 2016

Armstrong Renata admitted punching Cole Miller in the back of the head in a random attack in January 2016 Justice Robert Gotterson agreed the original sentence of seven years was "manifestly inadequate"

Justice Robert Gotterson agreed the original sentence of seven years was "manifestly inadequate" The Court of Appeal ruled Renata must serve 80 per cent of the revised nine years and six months jail term

Queensland Attorney-General Yvette D'Ath filed an appeal against the seven-year jail term given to Armstrong Renata after he pleaded guilty in the Brisbane Supreme Court to unlawful striking causing death.

Renata admitted to punching Mr Miller in the back of the head in a random attack while on a night out in the party precinct in January 2016, causing the teenager to fall to the ground and hit his head on the ground.

In his published reasons for changing Renata's jail term, Justice Robert Gotterson agreed the original sentence was "manifestly inadequate".

"The respondent delivered a very forceful blow to the deceased's jaw with a clenched fist. The blow was entirely unprovoked," he said.

"The deceased had his arms by his side. He had said nothing to the respondent or his associates.

"Moreover, the blow was delivered from out of the deceased's sight. The deceased had no opportunity to defend himself.

"It is no understatement to say that this is a chilling example of the cowardly, vicious conduct that s 314A was intended to address."

Justice Gotterson said taking into account Renata's "youth, his plea of guilty and the fact that he spent time in maximum security", a sentence of nine years and six months was appropriate.

The Court of Appeal ruled Renata must serve 80 per cent of the revised jail term.

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

Acting Attorney-General Stirling Hinchliffe said in a statement that he welcomed the court's decision to increase Renata's sentence.

"Cole's life was tragically cut short and I know his death was felt by Queenslanders everywhere," Mr Hinchliffe said.

"I am pleased the court recognised Cole's killer deserved a tougher sentence to reflect his cowardly and vicious act."

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.