George Patel went to court yesterday hoping the man who killed his elderly father after a road-rage incident would be jailed for a long time.

Instead, he watched as Bio O'Brien was given a three-year jail term for the manslaughter of Jasmatbhai Patel, 78 - which means he could be freed early next year when he becomes eligible for parole after serving one-third of his sentence.

The 28-year-old could be released then because he has already spent nearly six months in custody.

The Patel family are devastated because they thought their father's life was worth more.

"I'm not happy with three years but what can you do? That's the law in New Zealand," George Patel said after the sentencing at the High Court at Auckland.

O'Brien became enraged after the April 7 collision, which caused a 30cm dent to his black BMW, and hauled Mr Patel from his van and beat him about the head and face.

He gripped him by the shirt, pulled him forward and then pushed him away, causing the pensioner to rock back and forth.

O'Brien then pushed him backwards on to the concrete and Mr Patel's head hit the curb, fatally injuring him.

O'Brien pleaded guilty to the manslaughter charge in May.

George Patel said he would never forgive O'Brien and doubted he was remorseful. O'Brien had wanted to meet the Patel family at a restorative justice conference but they refused.

Mr Patel wanted a longer jail term but said he wouldn't have been satisfied with any punishment for what his father had to endure.

"[For] my father's life ... any sentence is not sufficient for us.

His voice cracking with emotion as he read his victim impact statement to the court, Mr Patel told of promising his mother 20 years ago he would look after his father for the rest of his life.

"I hoped he would outlive me. He was disciplined and led a healthy, active life. I feel like I have lost a precious part of my life."

Justice Judith Potter set a starting point for sentence at 4 years but reduced it to 18 months in recognition of O'Brien's early guilty plea.

She noted his desire to meet the family and his promise to complete an anger management course in jail, which he would have to do after a probation report found he was at a high risk of offending but was highly motivated to change.

However, the judge said the attack was an appalling case of road rage. All motorists occasionally became frustrated but his reaction had been disproportionate, she said.

"The assault was unprovoked and unjustified."

O'Brien has offended before. In 2001, in Dunedin, he was ordered to complete 150 hours community service for threatening to kill and for assaulting a female.

* Manslaughter - big differences

Sentencing guidelines are not as strict for manslaughter cases.

Five people found criminally responsible for the death of Wainuiomata woman Janet Moses this year received community-based sentences. Ms Moses drowned after a Maori curse-lifting ceremony.

Bruce Emery was jailed for 10 years when he was convicted of manslaughter in December, 2008. He had been charged with the murder of Pihema Cameron but a jury found him not guilty of that charge but guilty of manslaughter.