Yesterday's attack on an officer at Christchurch Prison has prompted their union to call for response units armed with tear gas and pepper spray.
The prison officers are not equipped or armed to deal with the increasing violence they are facing and need back-up, said Corrections Association presidentBeven Hanlon.
In yesterday's incident, prison managers were warned that inmates in a 60-bed unit were planning an attack hours before the officer was assaulted, with guards outnumbered by 20 to one, the union said.
The three guards were told to patrol the unit together, unarmed.
The Dominion Post reported that a female guard, who is about 158cm (5ft 2in) tall, was king-hit from behind as the three guards made their way through the unit. She lost a tooth and needed stitches above her eye.
The inmate then turned on the other guards before he was restrained.
"Three people in just their cotton uniforms aren't going to be able to do a lot if (60 prisoners) decide to attack them," Mr Hanlon said.
"We've had Corrections officers raped, stabbed, beaten, taken hostage, you name it, except for murder. Anything could have happened."
The attack follows another two weeks ago on two Rimutaka guards who were beaten by a mob of inmates in an assault that left them covered in blood.
Mr Hanlon said the prison had failed to ensure the guard's safety.
"If police had information that one of their officers was going to be attacked, they would arm their officers with firearms, bullet-proof vests, pepper spray. They would take all the precautions necessary.
"We're saying this is unacceptable."
Ian Bourke, Corrections Department southern assistant regional manager, said when staff were warned of the assault, every precaution was taken to ensure the guards' safety.
"While locking the unit down was an option, this is likely to have made the situation worse because prisoners would have become more angry and frustrated. This would have increased the risk of an assault occurring when the unit was unlocked."
Mr Bourke said Corrections was already undertaking a project to assess the extra protection needed by staff and met the union last week to discuss their concerns.
