Two prison guards have been hurt in an attack at Christchurch Men's Prison, in one of two attacks on prison officers on Saturday, the Corrections Association says.
The association, which represents guards, says a prisoner refused to leave a staff office area when told and turned on a guard who tried to usher him out of the office.
It says the prisoner punched the officer in the head several times, then put him in a headlock.
A woman officer grabbed the prisoner's arm and hit a concrete wall as the three crashed to the ground.
She told her union the unit involved was short staffed at the time and the incident could have been prevented if the guards were allowed to carry pepper spray.
The woman officer was first thought to have broken or dislocated her arm but an X-Ray suggests that's not the case.
The association says another assault took place at Auckland Women's Prison when a prisoner punched a woman officer in the head.
The Corrections Department says staff will soon have access to pepper spray now that a 12 month trial has been completed.