A Mohaka man who shot dead his son at their shared home in the remote northern Hawke's Bay township last year, has become the ninth person in New Zealand to avoid life imprisonment for murder.

Dean Cole, 62, was sentenced by Justice Helen Cull in the High Court at Gisborne to 12 years imprisonment with a minimum term to be served of six years.

He had pleaded guilty in February to murdering Blair Cole, 42, last October.

Defence counsel Susan Hughes QC successfully argued that following the legislation (revised in 2002), to sentence Dean Cole to life imprisonment, would be manifestly unjust. He was suffering a serious, long-term mental illness and was provoked by his son who had a reign over terror over his father and his family spanning two decades.

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Dean Cole's solution to the problem was not one a person in their right mind might have arrived at but he was not in his right mind, Hughes argued.

He and his family had been physically abused, some suffering dramatic injuries, that caused them to distance themselves from Blair Cole.

Dean Cole was trying to cope in isolation with his son whose violence was often fuelled by methamphetamine and alcohol.

In the two months since Blair Cole went to live with him, threats had escalated. Blair Cole was always angry, believing that his father had ripped him off over the settlement of the property, which was his grandmother's, before she died in 2015.

Hughes sought a sentence of nine years with no minimum term.

Prosecutor Steve Manning submitted that even if the court was minded to step back from a life sentence, the offence required a minimum term to be served.

Dean Cole shot his son in the chest once in an "execution-style" shooting from less than a metre and a half away, Manning said.

The younger man had been sitting on his bed mid-morning reading mail and oblivious to his father in the doorway with a borrowed .22 rifle.

Many people who committed murders had mental illness, the threshold for a reprieve from life imprisonment was extremely high. Dean Cole's circumstances did not meet it. His concern for his other children was unwarranted; they lived overseas.

Blair Cole had not directed any violence or threats at him that day, Manning said.

Justice Hull noted that immediately after firing the shot, Dean Cole threw his son a towel and told him to apply pressure to the wound. When he realised it was fatal, he phoned the authorities, who he spoke with frankly, and waited at home for police and ambulance to arrive.

Blair Cole was pronounced dead at the scene.

Since the shooting, members of the Cole family had corroborated Dean Cole's accounts of a disgruntled, violent Blair Cole.

The night before the incident, he had threatened his father, saying he would kill him and bury him in a swamp, then go after his brother and sister, who planned visits for the near future.

Dean Cole told a 111 operator immediately after the shooting that he had planned to put an end to his son's "reign of terror" by "wheelchairing" the younger man, and had not intended to kill him. His actions were designed only to incapacitate his son so that he could not carry out his threats.

Dean Cole told the operator he was sick of his son's behaviour, that he had asked for help in the past but never got any, no help from mental health services and that he had now ended the misery.

In the wake of those comments, the officer in charge of the case, Detective Sergeant Daniel Kirk of Wairoa Police said he wanted the public to know police took domestic violence very seriously. He urged anyone who needed help to go to police before considering taking matters into their own hands.

This case proved the courts dealt with all cases as uniquely but that there were never any winners in murder - only losers.

In this case a family was left grieving the death of a son and sibling and the long-term incarceration of a partner and father.

Dean Cole's family is standing by him. He

Cole was supported by his ex-wife, two children and other family members at his sentencing at the High Court in Gisborne.

Hughes said the whole family is happy with today's sentence.

"Like him, [they are] pleased that he has received a sentence of less than life," the defence lawyer said.

"They understood the process. We've obviously discussed it in the months leading up to today."

During the two-hour sentencing hearing, the court was told the eight other cases where reprieves from life imprisonment for murder was granted involved mercy killing, battered defendants who had been subject to severe and prolonged abuse, a major psychotic offender, someone who was not the principal offender, and an extremely young offender.

- Gisborne Herald, additional reporting from Newstalk ZB