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Fire has damaged a technical block at Auckland's Otahuhu College.

More than 40 firefighters rushed to the college about 11am today when the fire broke out in the roof space of the school's old technical block.

No one was hurt and the fire was extinguished in a short time.

When firefighters arrived the ceiling space was burning fiercely and they initially had trouble finding the source, said deputy chief fire officer for the Auckland east area, Roger Callister.

He said there was a false ceiling "which gave us a few challenges".

"The fire was burning in an area within the ceiling void itself and we had to break through.

"'Once we broke through the fire popped out in a couple of places and we managed to make a bit of headway."

He said the fire damage was contained to two classrooms although there was some minor smoke and water damage to other classrooms.

"The fire trucks...managed successfully to prevent it extending to the other six classrooms," Mr Callister said.

He said the building had no sprinklers and firefighters had trouble getting a good water supply until more fire trucks arrived and began pumping water from the street.