Acute mental health nurses at Christchurch's Hillmorton Hospital are being assaulted by patients at least twice a week, and as often as once a day.
Some were so fearful of assault that they spent their own money on food and cigarettes for patients to placate them, one nurse said.
Isolated assaults at the hospital have been widely reported, such as an attack two months ago that left three nurses with serious injuries.
But some of the the 150 nurses at the inpatient mental health unit said they were being assaulted on a regular basis and not all incidents were being accurately recorded.
Canterbury District Health Board chief executive David Meates confirmed assaults on staff were commonplace in the unit.
In the past 12 months alone, the frequency ranged from eight to 30 recorded assaults each month.
One of several nurses spoken to by RNZ said the true number was even higher, with about 100 incident reports filed each month.
The reports recorded every incident in the unit, whether it was an assault or even just having to restrain someone.
The nurse, who did not want to be named, said some nurses were worried assaults were not being accurately recorded.
"The feeling on the floor is that the assaults that we are reporting don't always make it onto the official stats [and] we fear that it is not getting past management," the nurse said.
"People outside mental health are not getting a clear picture of the extend of the assaults in the hospital."
Some patients who had been given permission to go on leave from the unit returned high on drugs or alcohol, the nurse said.
"Trying to get a urine drug test off some of these guys is completely impossible unless you want to be assaulted."
To protect themselves from those patients, some nurses went to extreme measures.
"Often you have to give into what the patient wants, otherwise you are going to get assaulted," the nurse said.
"I know nurses who will go and buy patients cigarettes and food out of their own pocket so they can feel safe at work."
One fix could be security officers placed in the wards, but staff were worried that would only happen in reaction to a serious incident, they said.
"There is an underlying fear among nurses in acutes that it's going to take someone dying before something is done about this," the nurse said.
CEO: 'We are deeply concerned'
Mr Meates said he did not consider it acceptable for anyone to be harmed at work and he was deeply concerned about the situation.
CDHB was trying to address the safety problems, included developing an eight-bed community-based acute alternative service, a $3.5 million redevelopment of the adult acute inpatient unit, and investigating changes to staffing.
The 64-bed unit could not cope with increased demand since the Canterbury earthquakes.
"We frequently have up to 90 patients in the unit during the day," Mr Meates said.
"This creates a situation with more people in a space that isn't designed to accommodate such large numbers and we are seeing more incidents as a result.
"We know our outdated facilities are contributing to some serious issues for us."
There were mixed views about the benefits of security officers being placed in wards, but CDHB was considering creating new jobs that would focus on "direct engagement and support for patients".
Asked about claims that assaults were not being accurately recorded, Mr Meates said both near misses and actual assaults were recorded and frequently reviewed.
If any staff member had any concerns, they needed to speak up, he said.
"If staff feel unable to do that directly it's important their union is involved."
New Zealand Nurses Organisation spokesperson John Miller said the union had met multiple times with the DHB and he was optimistic change was coming.
However, the union wanted an audit of the assault statistics to ensure that they were accurate.