A prisoner has been jailed for seven years for slitting the throat of a fellow inmate - and he will serve every day of the sentence.

Anthony John Wheble (24) was originally serving a four-and-a-half-year term at the Otago Corrections Facility for an aggravated robbery and assault with intent to rob, for which he was sentenced in 2015.

With a strike to his name, the attack on 33-year-old Brazilian prisoner Diego Marques-Santos, using a razor blade fused to a piece of plastic, represented his second strike.

Justice Kit Toogood in the High Court at Auckland yesterday confirmed Wheble would be required by law to serve the totality of his new sentence, which would be tacked on to the one he was serving.

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However, the judge took aim at the three-strikes legislation.

''I cannot help but observe that your case proves that there has been an abject failure of the three strikes regime in acting as a deterrent. But I have to apply the law as it is,'' he said.

On the morning of October 20, the prisoners in Wing H were released from their cells.

Wheble ''stalked'' his victim around the day room, court documents said, then grabbed him from behind, using his right hand to inflict an 8cm cut to his throat, which narrowly missed the carotid artery.

Marques-Santos lowered his head to protect himself while Wheble tried to slash him twice more with the implement.

After others intervened, Wheble twice stomped on the victim's head as he lay in the recovery position.

He then kicked the man's head at least five times ''as if kicking a football''.

Marques-Santos was airlifted to hospital, where he received 15 stitches to his neck as well as treatment for a nasal fracture and severe bruising.

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He told the Otago Daily Times he was "alive by a divine miracle".

"I was saved by another prisoner who quickly blocked the bleeding by putting his top around my neck ... I was choking on my own blood," he said.

Marques-Santos is serving a sentence of three-and-a-half years after targeting gay men through dating app Grindr, drugging them and stealing their bank cards.

He ripped off men in Dunedin and Nelson for $10,000.

Justice Toogood described the prison incident as "extreme".

"This was an unprovoked attack, without warning and from behind. Mr Marques had no opportunity to defend himself."

"The slashing of Mr Marques' throat with a razor blade could well have killed him, and he might have died from any one of the kicks to the head ... but Mr Marques recovered relatively swiftly. He remains physically and emotionally scarred, however."

Wheble had a significant list of previous convictions that had got progressively more serious, the judge noted.

The defendant's background somewhat explained his spiral.

"You are a paradigm example of the historic failure of our welfare and criminal justice systems to help young offenders."

Wheble was supported by letters from his mother and other female relatives, the court heard.

They wrote about the trauma he had suffered in his life and the potential he showed.

"You are an accomplished artist and you have written poetry and rap," the judge said.

Wheble was assessed as a very high risk of further violent offending but Justice Toogood stressed he was still young.

"It is not too late for you to turn things around," he said.

"But to do that you will need to overcome your hostility to authority and accept the help which I hope will be available from the highly committed psychologists and other experts who work within our prisons."