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The grieving family of school principal and councillor Hawea Vercoe are petitioning to have his killer's prison sentence extended to 20 years.

They say 34 months is not enough jail time for Isaiah Tai, 21, who was sentenced for manslaughter three weeks ago.

Mr Vercoe, 36, died after being punched in the back of the head by Tai outside a Whakatane bar in November. Tai forcefully kicked him in the head as he lay unconscious on the ground. Mr Vercoe died later in hospital.

His aunt Rihi Vercoe said the collection of signatures for a petition would be part of a nationwide appeal for a mandatory sentence of 20 years for taking a person's life.

She felt the sentence had cheapened the life of an aspiring Maori leader and father of six.

The family will send the petition with a letter to the Crown Law Office and the Attorney-General, encouraging an appeal.

She told the Herald that New Zealand's justice system was angled too strongly at supporting criminals, not the families of victims.

"We have been sentenced with a life term ... because the court is not sharply focused on those who are suffering."

In sentencing Tai, Judge Judith Potter took into account Tai's early guilty plea and participation in a restorative justice session with the Vercoe family.

Vercoe's mother Rosalind said after Tai's sentencing that he lacked contrition, and had seemingly only attended the meeting to "save his skin".

"I don't know how sincere he is," she said.

The Vercoe family said the "joke" of a sentence had been worsened by Tai's family's behaviour at court. They felt Tai's supporters' lack of sincerity mirrored their son's failure to show remorse.

Tai's appearances in the Rotorua High Court were marred by ugly scuffles between the families. Cheers from Tai's family when he was sentenced led to a heated confrontation outside the court.

Crown prosecutor Greg Hollister-Jones had called for six years at the sentencing, to deter alcohol-fuelled street violence. He said Tai's assault was "unprovoked and gratuitous" and had robbed six children of a father.

Mr Hollister-Jones told the Herald the file had now been referred to the Crown Law Office in Wellington for consideration for an appeal.

Pleas or petitions from families of a victim do not influence an appeal. The Deputy Solicitor-General's decision on the appeal is based on comparisons with past sentences. But Rihi Vercoe said she believed the "injustice" of the sentence would be scrutinised.