Worries a gathering crowd would steal their equipment saw fire services call in police to stand guard over trucks as fire fighters battled a blaze in Wellington last night.

The fire was one of three in the region overnight, with 23 fire appliances called out to battle a house and a cow shed on fire as well.

The most serious of the three was the block of flats catching fire around midnight in Aro Valley, an inner city suburb of Wellington.

The fire occurred in the bottom flat of a four-storey building in Devon Street and 14 fire trucks attended.

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Because the street was so narrow the trucks were parked about 200m away from the flats, Assistant Area Commander Scott Bell said.

"There was a crowd gathering and we were concerned for the safety of some of our equipment."

Police were called to guard the trucks shortly after midnight and Bell said he did not believe anything had been stolen.

"It was more of a preventative measure," he said.

"Based on some previous trouble around the rest of the country, particularly in student areas, it is a concern we had."

Bell was unable to confirm reports items, including road cones, had been stolen, saying he wasn't aware anything had actually been taken.

The fire went to a fourth alarm, indicating it was a serious incident, a fire service spokeswoman said. No one was injured.

Earlier in the night, firefighters were called to a house fire in Tawa, just north of Wellington city.

Six appliances attended. There were no reports of injuries.

Around 3am, three Wellington fire appliances and a rural unit were called to a cow shed that had caught fire with young calves inside.

No calves were injured, a fire service spokeswoman said.