It took just 10 seconds for Morgan Alexander Isles-English to end his abusive father's life with a single blow to the head after his dad didn't recognise him.
With time served in pre-sentence custody, the now remorseful Isles-English, 27, will spend at least the next five years in prison.
As she sentenced him on Wednesday, Justice Soraya Ryan said the troubled son didn't mean to kill his father, Mark English, in July 2016.
Isles-English hadn't seen Mr English for about four years on the day he struck the fatal blow.
He spotted his father and new wife, Varri Telfer, at the Anzac memorial in Brisbane while drinking at a pub and crossed a road to confront him.
"The deceased apparently not recognising you said 'who are you mate, who are you?'," Justice Ryan told the Brisbane Supreme Court.
"You said 'yeah, bet you didn't expect you'd see me again'."
"When he turned to look at you, you struck him with a closed fist to his cheek," she said, addressing Isles-English.
"It's not suggested the punch itself involved particular force but it caused the deceased to fall backward and his head landed on the concrete."
Mr English died in hospital three days later devastating his wife, Ms Telfer, who witnessed the blow, Justice Ryan said.
"In her statement, she speaks of her love for the deceased and how devastated and hopeless she feels after his death," she said.
"Without his financial support she became homeless and she will forever be haunted of her memory of July 4, 2016."
Isles-English saw his victim in a different light - as a drug and child abuser.
"In a letter to the court your mother, who has offered you her unfailing support, ... explains you received little positive attention or praise from him," Justice Ryan said.
"You endured his physical violence and emotional and verbal abuse, even though you were just a child."
Isles-English's mother left Mr English after a "particularly violent attack" on Isles-English when he was seven years old, she said.
"Your father told you he never wanted you or you brother, and he said other cruel and hurtful things to you," Justice Ryan said.
"I acknowledge the man known to his new partner and her children was a very different man to the man known to you."
Isles-English was sentenced to a maximum eight years in prison for a string of offences, including unlawful striking causing the death.
Justice Ryan said her sentence took into account Isles-English's poor relationship with his father, that he was remorseful and the blow wasn't "particularly severe".