An inmate tampering with an electrical socket in his cell caused a fire at Mount Eden prison in Auckland last night.
Corrections said staff removed the inmate from his cell and called Fire and Emergency.
The fire was contained to the cell and no staff or prisoners were injured.
Because of the smoke caused by the fire, the remaining prisoners in the unit were evacuated and secured by staff.
They were returned to their cells just after midnight, after extractor fans cleared the remaining smoke.