Inmates at Ngawha Prison have broken into a unit control room, prompting a union spokesman to describe the facility as a powder keg ready to explode.
The Department of Corrections has acknowledged prisoners were able to access the control room of one unit on one occasion but said there was
no possibility of them escaping and staff were not put at risk.
The department adds that comments suggesting the prison is unsafe or at boiling point are alarmist.
Corrections Association president Beven Hanlon said a group of prisoners broke into a control room in the remand unit last month and stole food stored there.
Prison staff were concerned about the breach because inmates could have seized control of the unit by taking hostage a staff member.
Prison Services acting general manager Leanne Field said staff had quickly identified how the breach occurred and took immediate steps to prevent it from occurring again.
"A unit control room does not allow prisoners to open the unit's main gates and there is no risk of prisoners being able to escape by gaining access."
A department spokeswoman said the unit control allowed staff to monitor activity but the main gates were controlled by a master control room outside the unit.
Mr Hanlon said the issue highlighted ongoing safety concerns at the prison.
Last week, the department said it had reviewed its security procedures at Ngawha Prison after an inmate was viciously bashed, despite the presence of surveillance cameras.
Matthew Te Hira was left with critical injuries after he was bashed in his cell by a group of prisoners in March last year.
The department said it had taken steps to improve security and safety, including buying extra security cameras for the prison.
Ms Field expressed absolute confidence in the staff and management on the site.
She said the department would be providing additional training for those staff working in high-risk situations.
