Prison officers at Auckland Prison at Paremoremo have lodged a vote of no confidence in the prison manager following serious attacks on staff.
The union says management is failing to take simple measures to protect personnel at the maximum security facility or address major staffing shortages.
Association national president Beven Hanlon says the concerns, which have been raised repeatedly with management, have fallen on deaf ears.
He says the situation is out of control, as prison guards are unable to protect themselves, let alone inmates.
Mr Hanlon says there are supposed to be three guards to every inmate, but in reality there is only one to every 12.
The Corrections Department on Wednesday says it has every confidence in the prison's management and guards need to take some responsibility for their safety.
Northern Regional Manager Warren Cummins says the department is discussing the situation with the union.