A Killer Beez inmate who shanked a rival gang member in retaliation for the death of his niece after she was raped by the Mongrel Mob has had his sentence reduced after the courts found the term of imprisonment was "manifestly unjust".

Stead Nuku, now 29, had just turned 27 and was a maximum-security prisoner at Auckland Prison, when he repeatedly "shanked" another inmate by stabbing him with an improvised blade during "fight training" in a prison exercise yard.

Nuku, a member of the Killer Beez gang, said the attack on the rival Mongrel Mob inmate was in retaliation for his niece's death after she was gang-raped by other members of the Mongrel Mob.

The injured man sustained more than 20 stab wounds to his face and body, including a three-centimetre gash to the back of his neck during the 2019 attack.

He now finds it difficult to turn his head, can no longer lie in certain positions, and has flashbacks and nightmares. He suffers anxiety around the potential of another attack.

Nuku pleaded guilty to wounding with intent to cause grievous bodily harm and was sentenced to 14 years jail under the three strikes laws.

These laws are designed to selectively incapacitate repeat offenders by mandating lengthy prison sentences for those convicted of particular types of crime more than once.

The shanking was Nuku's fourth "strike" while behind bars, he had committed other strike offences when he was aged 23, 24 and 25.

He didn't challenge the preventive detention aspect of the sentence but his counsel Emma Priest contended it should have only been for 10 years.

She argued the sentencing judge erred by failing to take adequate account of the sentencing purpose of assisting Nuku's rehabilitation and reintegration, by failing to give sufficient discount for personal and cultural mitigating factors and by failing to give a full 25 per cent discount for Nuku's early guilty plea.

The Court of Appeal has this week agreed on the first two grounds, particularly on the discount for personal and cultural factors, which it upped from 6 per cent to 15 per cent.

It removed a 12-month uplift the sentencing judge gave for Nuku's previous offending, saying it was not necessary given the earlier three-strikes sentences imposed.

But the court found the 20 per cent discount for a guilty plea in the original sentencing was within range because of the overwhelming evidence against Nuku.

The 2019 attack happened in Auckland Prison.

It noted the sentencing judge's comment that "in the absence of the three strikes regime", she would have imposed a sentence of 10 years, six months' imprisonment for which the two-thirds minimum non-parole period of seven years would have applied.

The court agreed with that finite sentence but reduced it to 10 years. Nuku must serve all of it.

It also noted the present offending was similar and occurred a comparatively short time after earlier offending. Unsurprisingly, the relevant risk assessment at the time of sentencing was therefore unchanged, the Court of Appeal said.

It noted the disparity between the 14-year mandatory non-parole period and the seven-year non-parole period of the last offence, was double.

To impose it would mean Nuku would be treated in the same way as an offender who had no personal mitigating factors reducing their culpability and had not accepted responsibility or pleaded guilty at an early stage.

Nuku was jailed for the first time at 17 and had been subject to a succession of cumulative prison terms ever since.

"Apart from supervision in the Youth Court, he has never received a rehabilitative sentence and it appears he has not had the benefit of any form of treatment, counselling or other assistance to address the causes of his offending," said Justice Murray Gilbert in a decision released this week.

"It would be counterproductive, and contrary to the public interest, to impose a minimum period of imprisonment on a relatively young man like Mr Nuku that is so crushing as to remove all hope of rehabilitation and eventual reintegration.

"As it stands, he has no prospect of being able to apply for parole until he is in his early 40s, irrespective of the outcome of any rehabilitative efforts made by him in the meantime. By then, he will have spent the better part of 25 years in prison."