A man who killed a Brisbane father in a one-punch attack on a South Brisbane footpath has been sentenced to eight and a half years in jail.

Key points: Wayne Andrew Hearn and Timothy Lang were hostile to one another before the attack

Wayne Andrew Hearn and Timothy Lang were hostile to one another before the attack Hearn believed Mr Lang had slept with his former partner

Hearn believed Mr Lang had slept with his former partner Hearn must serve at least 80 per cent of his sentence in jail

Wayne Andrew Hearn pleaded guilty to unlawful striking causing death in the Supreme Court in Brisbane.

The 40-year-old punched Timothy Lang in the head after the pair, who knew each other, crossed paths on Melbourne Street in October 2016, knocking him unconscious.

Mr Lang, 48, fell and hit his head on the concrete, suffering severe trauma.

He was taken to the Princess Alexandra Hospital but died two days later.

The court heard the two men knew one another and there was hostility between them because Hearn believed Mr Lang had slept with an ex-partner.

Mr Lang said something to attract Hearn's attention as he jogged past, but prosecutor David Nardone said it was not clear what Mr Lang said before he was attacked.

Mr Nardone said Hearn leant over Mr Lang after punching him and said: "I told you to stop. You wouldn't let it go and that's what you get."

Hearn then walked off and asked a member of the public to call an ambulance.

Justice Peter Applegarth described Mr Lang's death as "tragic".

"Whatever his troubles in life, (he) deserved better than to die on a street," he said.

Hearn read a letter apologising for his crime. ( ABC News )

"Whatever he said to you, may have been somewhat provocative, however his verbal provocation did not excuse your violent reaction, you overreacted."

But Justice Applegarth said Hearn's violence was "impulsive" and not premeditated.

"You said you didn't realise how hard you hit him.

"You also told police you felt bad about what had happened and bad for the deceased's family."

Hearn must serve 80 per cent of his sentence behind bars before being eligible for parole.

The prosecution said Hearn had an extensive criminal history, including random acts of violence against people unknown to him.

He was on parole at the time of his attack on Mr Lang.

Hearn read out an apology letter from the dock and said his actions were unacceptable and devastating.

"Time cannot be turned back and now both our families are left with these tragic consequences," he said.

"I take full responsibility for what occurred and I hope one day Mr Lang's family and society can forgive me."

Defence counsel said Hearn was jealous and upset but never intended to kill Mr Lang.

The court heard he was subjected to horrendous events from a young age which led to a desensitisation to violence.