UPDATE 12.25PM:A third deliberate fire in Tauranga Hospital's Acute Mental Health Unit, in the space of a month, has officials pondering how to put an end to the dangerous actions.

Firefighters and police were called to the hospital's unit on 20th Avenue about 5.45pm on Sunday after a sprinkler activation forced a full evacuation of the unit.

Police arrest a man following the fire in Tauranga Hospital's Acute Mental Health Unit. Photos: Cameron Avery.

Tauranga Fire Brigade senior station officer Len Sabin says it was a small fire in one of the rooms, similar to a deliberate fire set in the unit on September 28 and another on Sunday October 12.

'It was a very similar thing except they had set some clothing on fire in the corner of one of the rooms which triggered a full evacuation of the building and the sprinklers activated and extinguished the fire.

The fire on September 28 was caused by a man setting fire to towels by placing them on top of a toaster – causing 20 per cent of the mental health unit to become smoke-logged.

Len says police and fire safety are continuing to investigate this latest incident with how the clothes were lit yet to be determined.

'Police are treating it as a crime scene.

'There wasn't a significant amount of damage but the potential was there again. Obviously on the back of what they had a month ago they are keen to have a good hard think about things.

Bay of Plenty District Health Board Mental Health and Addiction Services clinical director Sue Mackersey says 33 patients and staff were evacuated and nobody was harmed in the incident with everyone returning to the building 45 minutes later.

'The room where the fire took place sustained smoke and fire damage. There was no damage to the remainder of the building and services have not been affected.

'The circumstances surrounding the fire are being investigated.”

Tauranga Police Senior Sergeant Carl Purcell says police are still making enquiries into the incident but have a man in question.

He says the man was trying to light the clothes on fire in his room of the hospital unit.

Later that night at 8.15pm the Tauranga firefighters were called to a Waitaha Road address in Welcome Bay where a light fitting had overheated causing a fire in the roof cavity.

Len says the roof cavity of the single storey house, being rented out, has a significant amount of damage which luckily didn't spread to the rest of the house.

Firefighters spent some time ended up taking off some roofing iron and removing insulation in order to get to the fire and light fitting.

'They [the tenants] noticed the light was flickering and could see it was burning up behind the light.

The cause is still being investigated, but we suspect they have put a new filament in and it's probably too strong for it [the light fitting].”