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.

It is understood he used the cuffs as a weapon during the violent incident at Whanganui Hospital late on Sunday night.

NZME has learnt that the two prison officers had escorted the inmate from prison to the hospital for treatment.

As they were getting him out of the van he allegedly turned on them.

A source said he had managed to get his hands out of the cuffs placed on him at the prison and use them to wound the officers.

One officer had part of his ear bitten off during the attack and the other needed a number of stitches and medical staples to close a gash to his head.

Police are investigating the incident and say it's likely the inmate will be charged.

Corrections acting Deputy National Commissioner Neil Beales confirmed the incident, however, as it was under investigation, he could not comment on the specifics.

"On Sunday evening a prisoner was escorted to Whanganui Hospital at approximately 11pm, following assessment by an on-site prison nurse," he said.

"On arrival at the hospital, the prisoner assaulted the staff who were escorting him.

"The staff responded immediately to restrain the prisoner with the assistance of hospital security, and police were called.

"The Corrections officers showed immense courage to bring the situation under control quickly and safely.

"In doing so they sustained injuries to their faces, heads and arms and one officer has a serious ear injury."

Beales said both officers were treated at hospital and were discharged soon after.

Both officers were being supported by Corrections.

"They have been visited by prison management and their colleagues and have been offered support through our Employment Assistance Programme," said Beales.

"Assaults on our staff are unacceptable.

"Every assault on a member of our staff is taken seriously and any prisoner who resorts to violence is held to account.

"Our staff do an incredible job in some very challenging circumstances to keep New Zealanders safe.

"Our people deal with some of our country's most dangerous and volatile people in a complex and challenging environment, where assaults by prisoners are often spontaneous and come without any warning."

Beales said the inmate was "immediately returned" to Whanganui Prison and was being transferred to maximum security at Auckland Prison.

"Police are investigating the incident and charges are likely to be laid against the prisoner," he said.

"Corrections is undertaking a full review of the incident.

"The reality is the threat of violence is something we cannot eliminate entirely, but we do everything possible to minimise this risk.

"We have invested significantly in training and tools to keep our people safe."

Those tools included tactical skills such as de-escalation, stab resistant body armour, on-body cameras and the expanded use of pepper spray.

A police spokesperson also confirmed charges were likely.

The investigation into the incident was ongoing and no further comment could be made.

In September (2018) Chief Ombudsman, Peter Boshier, warned urgent action was needed at Whanganui Prison to stop violence and intimidation going on there.

Following a spontaneous inspection, it was found two of Whanganui Prison's units had the highest number of recorded assaults of all the Department of Corrections' lower north region facilities.

The Chief Ombudsman said it was surprising the prison didn't have a gang management strategy in place last year, considering almost half of prisoners were either gang members or associates.

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But, Boshier did find interactions between staff and prisoners were generally positive.

Corrections Association spokesman Beven Hanlon said prison staff dealt with attacks by inmates "on a daily basis".

"Our thoughts are with the injured staff who did nothing wrong but acted with extreme bravery when attacked with a weapon to stop this prisoner escaping and becoming a problem for the public," Hanlon said.

According to Corrections' latest annual report for 2017/18 there were 12 "serious" assaults on prison staff by inmates - which equates to a rate of 0.11 per
100 prisoners.

From the 2018/19 financial year, Corrections moved from reporting numbers of serious prisoner on prisoner and serious prisoner on staff assaults to reporting each as a rate per 100 prisoners.

They claimed rates were considered to be "a more accurate way to track incident trends over time as they allow for fluctuations in the prison population".