A prisoner remains in hospital after an alleged assault in Whanganui Prison.
The prisoner has been recovering in hospital since the incident on February 19 which police are now investigating.
Whanganui Prison director Reti Pearse said Corrections had a zero tolerance for violence policy and any violence or assaults against other prisoners or staff was not tolerated.
"Any prisoner using such behaviour will be held to account for their actions, including facing criminal charges," he said.
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"We are constantly working to ensure our prisons provide the safest environment possible for staff and prisoners."
The latest incident follows one in January where a Whanganui prison officer had part of his ear bitten off and another needed staples and stitches to a serious head wound after an inmate they were transporting to hospital slipped out of his high-security handcuffs and attacked them.
Last year a report by Chief Ombudsman Peter Boshier said urgent action was needed to stop violence and intimidation at the prison.
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.