A Northland man who attempted to murder his mother with a hammer will serve an extra two years in jail before becoming eligible for parole, the Court of Appeal has ruled.

In a written finding, the court said Haden Karl Brown, now 22, would serve a minimum five years' non-parole time in jail.

Brown was sentenced in the High Court at Whangarei to nine years' jail on July 3, meaning he was eligible for parole after three years.

The Court of Appeal said three years was not long enough.

"We have no doubt that the circumstances of this offence of attempted murder are sufficiently serious to justify a minimum period of imprisonment greater than three years," it said.

Five judges heard the appeal, and there was much debate between them and lawyers over the wording and intent of the Sentencing Act 2002.

Brown bashed his mother Sue Brown with a hammer at her Hukerenui home on June 30 last year. He also admitted a charge of arson.

The attack left Brown's mother with brain damage and ended her teaching career.

At the time of the attack, Brown had a chronic depressive disorder.

The Solicitor General did not appeal against the nine years' jail Brown had received for his crimes, but said the sentencing judge did not include a minimum non-parole period.

Crown lawyer John Pike said the sentencing judge, Justice Peter Salmon, had the power to impose a minimum non-parole period under the Sentencing Act.

"Mr Pike contended that the sentencer must consider whether the seriousness of the offence calls for a non-parole period of more than one-third of the nominal sentence," the court said.

Although Brown's crimes were committed before the Sentencing Act came into force, it applied because his offence was a "serious violent offence", the court said.

In sentencing Brown, Justice Salmon "plainly had in mind" the provisions of the Parole Act, the court said.

Defence lawyer Gerard Winter said it was for the Parole Board to determine whether Brown would present a risk to the community if released after three years.

The Sentencing Act should not be used to deny "the right to parole where there is no danger to the community".

The court further noted that it was difficult to assess whether Brown was likely to offend again and that he should have "considerable time" spent on treating his disorder.

"There is no issue in this case of mental illness obscuring from the offender the true nature and seriousness of the criminal acts," the court said.