A man who shot dead his son at their Mohaka home in Northern Hawke's Bay has become the ninth person in New Zealand to avoid life imprisonment for murder.

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

In February he had pleaded guilty to the charge of murdering 42-year-old son Blair Cole on the morning of October 12 last year.

Defence counsel Susan Hughes QC, of New Plymouth, yesterday successfully argued that as per the provisions of legislation revised in 2002, to sentence Cole to life would be manifestly unjust.

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Cole was suffering a serious and long-term mental illness and was provoked by his son who had a reign over terror over him and his family that spanned two decades, said Ms Hughes.

His solution was not one a person in their right mind might have arrived at but she said Cole was not in his right mind.

He and his family had been physically abused, some suffering dramatic injuries, that caused them to distance themselves from Blair, who had been living with his father for about two months after moving from Auckland to the remote farm area about 35km south of Wairoa.

Cole was trying to cope in isolation with his son whose violence was often fuelled by methamphetamine and alcohol, and threats had escalated with Blair Cole angry in his perception his father had ripped him off over the settlement of the property, which was his grandmother's, before she died in 2015.

Ms Hughes sought an end sentence of nine years with no minimum term, but Crown prosecutor Steve Manning, of Napier, submitted that even if the court were 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, Mr Manning said.

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

Mr Manning said many people who committed murders had mental illness, but the threshold for a reprieve from life imprisonment was extremely high. Cole's circumstances did not meet it.

His concern for his other children was unwarranted - they lived overseas, the prosecutor argued, adding Blair Cole had not directed any violence or threats at him that day.

Justice Cull noted that immediately after firing the shot, Cole threw his son a towel and told him to apply pressure to the wound. When he realised it was fatal, he phoned 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 to police of a disgruntled and violent Blair Cole.

The night before the incident, he had choker-held and 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 who took his call 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 murder him. His actions were designed only to incapacitate his son so that he could not carry out his threats.

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

Police officer-in-charge Detective Sergeant Daniel Kirk, of Wairoa, told the Gisborne Herald 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 unique 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.

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.

Ms Hughes said later Dean Cole was a "battered defendant" and the judge sentenced him fairly.

"Battered defendants from time to time engage in pre-emptive strikes, understanding if they're not being beaten today or abused today, then tomorrow they will be," she said.

"And the law allows a judge to look at such a set of circumstances with compassion."

Cole's ex-wife, two children and other family members were in the court and were happy with the sentence, Ms Hughes said, adding: "Like him, pleased that he has received a sentence of less than life. They understood the process. We've obviously discussed it in the months leading up to today."

It was not the first such sentence for a Hawke's Bay murder. In 2010, a Napier woman was sentenced to eight years after being found guilty of murder in the stabbing of her partner of 17 years in suburban Tamatea the previous year. At that time Justice John Wild said it would be manifestly unjust to sentence her to life due to her "history of victimhood''.