A Canterbury District Health Board spokeswoman has confirmed an incident occurred at the hospital, which resulted in a staff member requiring medical treatment.

A mental health patient smashed through a secure door, breaking a nurse's thumb, as two other nurses were assaulted nearby.

Graphic photos posted to a nursing Facebook page show the nurse's thumb covered in blood and missing its nail.

A nurse at Christchurch's Hillmorton Hospital, speaking on the condition of anonymity, said the violence erupted shortly after 9pm on Wednesday.

FACEBOOK The nurse's thumb was broken trying to stop a patient breaking out of a secure room at Hillmorton Hospital.

A patient in the hospital's adult acute inpatient unit, Te Awakura, assaulted two nurses near a locked high-care area, the nurse said.

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This agitated another patient, whom staff believed was secure in a locked room. However, this patient managed to smash their way out of the room, causing "massive damage to the doors", breaking the glass windows and forcing their way out into the ward.

In the chaos, one nurse had her thumb broken and nail ripped out after it was caught in the door, the nurse said.

"These are meant to be secure, high-care safe areas. Well, last night it proved how unsafe those doors are," the nurse said.

Fearing for their safety, staff members fled to an office, locked themselves in, and called the police.

The nurse said the incident was serious, but assaults by patients against staff were relatively common at the hospital, as was staff going on ACC due to their injuries.

"We as staff, we don't feel protected any day we go to work at the moment," the nurse said.

"Nurses are becoming patients. It's just not OK, it's wrong."

A post to the "New Zealand, Please Hear Our Voice" Facebook page shared photos of the nurse's thumb injuries and called on other nurses to tell their own stories in the wake of the incident.

"We are sick to death of being hurt, or witnessing our teammates, dear friends, our partners and our family members being hurt while doing their underpaid and dangerously understaffed healthcare jobs."

A police spokeswoman confirmed emergency services were called to an incident involving staff and patients about 9.20pm on Wednesday.

One person was arrested and charged with wilful damage.

A Canterbury District Health Board (CDHB) spokeswoman confirmed an incident occurred at the hospital which resulted in a staff member requiring medical treatment.

"We are supporting all people involved and there will be a full clinical review of the event."

Last month, Stuff reported five staff seriously assaulted by patients in a secure unit at Hillmorton Hospital had together been off work for more than 1150 days due to their injuries.

Their injuries include a laceration, shoulder injuries, concussion and a sprain.

In March, three nurses were taken to Christchurch Hospital's emergency department for treatment after a patient in a drug-induced psychosis lashed out.

One of the nurses was "beaten senseless" and another was hit on the jaw line.

SAFETY STRUGGLE IN OVERCROWDED WARDS

Hillmorton has been running at or over capacity for more than two years. In December, occupancy was at 102 per cent, meaning patients had to "sleepover" elsewhere 41 times.

Occupancy was at 97 per cent in January and 96 per cent in February, above the optimal level of 85 per cent.

Stuff reported in February that safety issues including patients lighting fires, assaults and drug use were plaguing Hillmorton's acute mental health inpatient units.

Staff said they struggled to keep everyone safe in wards overcrowded with seriously unwell people.

It was not unusual to have half a dozen police staff on site to help, they said.

In December, a patient lit a fire, which sent four people to hospital with smoke inhalation and led to a 30-minute building evacuation. A month later the facility was evacuated again after another patient-lit fire.

Stuff reported last year that burnt-out mental health staff at Hillmorton went to work anxious and afraid for the safety of themselves and their patients, with assaults by patients escalating to an average of more than two a day.