An inmate who suffered life-threatening injuries in a brutal attack at Christchurch Men's Prison is still in hospital a month on from the incident.

The inmate was attacked by another prisoner on March 24.

The Herald has learned that the injured man remains in hospital more than a month after the bashing.

He was in a coma for several weeks and a source said he had to be resuscitated twice on his way to hospital.

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Corrections could not comment on the specifics of the attack or the injured inmate.

"This prisoner's condition has now improved," a spokesperson said.

"He is not in a coma and is currently recovering in hospital.

"The incident was referred to police at the time and they are currently investigating."

After the attack Corrections told the Herald that prison staff "responded promptly".

"We have a zero tolerance for violence policy, and any violence or assaults against other prisoners or staff is not tolerated in prisons," said prison director John Roper.

"Any prisoner using such behaviour will be held to account for their actions, including facing criminal charges."

It costs about $1000 per day to keep a patient in hospital in New Zealand in a general medical ward.

The costs increase to $3100 per day for a high dependency unit and $5000 per day for intensive care.