An inmate who was stabbed in the chest in Rimutaka Prison is still in hospital nearly a month after the attack.

A Corrections spokesperson said in a statement a prisoner was taken to hospital on April 28 after an "alleged assault" at the prison in Upper Hutt.

"The area in which the assault took place was secured as a crime scene and all evidence was provided to police, who are investigating. Corrections is also carrying out a full investigation into the incident."

Corrections said the prisoner was still going through treatment and recovery in hospital, but would not say what condition he was in.

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"Following the incident two prisoners were placed on directed segregation and moved to the prison's management unit."

Two men have been charged over the incident and are due in court this week.

A 37-year-old and 31-year-old have been jointly charged with causing grievous bodily harm against the inmate by stabbing him in the chest, and with injuring him by elbowing him in the head.

The 31-year-old is also charged with kicking the man in the head.

"As our investigation into the assault is still in progress, and for privacy reasons, no further information will be provided at this stage," the Corrections spokesperson said.

"We have a zero tolerance for violence policy, and any violence or assaults against other prisoners or staff is not tolerated in prisons. Any prisoner using such behaviour will be held to account for their actions, including facing criminal charges.

"We are constantly working to ensure our prisons provide the safest environment possible for staff and prisoners. Our staff manage some of New Zealand's most dangerous people in a complex and challenging environment.

"Over 75 per cent of the prison population have convictions for violence in their offending histories, and more than 90 per cent have had a lifetime diagnosis of a mental health or substance abuse disorder."