Investigators say a fire at a prison in Western Australia that forced detainees and staff to evacuate was caused by a faulty washing machine.

Fire crews were called to the privately-run Acacia Prison, east of Perth, on Wednesday night after a blaze broke out in an industries block.

More than 100 prisoners in nearby accommodation units were moved to the centre's gymnasium as a precaution.

The Department of Justice said while the fire caused extensive damage to the affected areas, no one was hurt.

A department spokeswoman said on Thursday fire and emergency services officers had indicated the blaze was accidental and had been caused by an electrical fault in a washing machine.