Emergency services were called to Auckland Girls' Grammar School after a fire broke out in the gymnasium.
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Fire and Emergency NZ reassured parents that nobody had been injured after two small blazes took hold on the first floor of the 20 by 50 metre building.
Eight fire crews with about 30 firefighters were working on the fire from Howe St and nearby Hopetoun St. The cause of the fire was unknown, shift commander Paul Radden said.
A firefighter at the school said backup units were called after the first units saw smoke.
He said damage was minimal, but the building would require smoke ventilation.
Police and fire investigators were working to establish whether the fire was suspicious.
A few children evacuated the building when the fire alarm sounded, but it was unclear whether they were involved, or just doing sports, he said.
Sheena Ross, who works nearby, said a fire engine was parked in the traffic on Hopetoun St, in Freemans Bay, and a firefighter was on the roof of the building in the school grounds.
She said smoke was coming from the building and that the presence of the emergency services was causing delays to traffic.
A police spokeswoman said police were called at 4.45pm and were assisting Fire and Emergency NZ at the scene on Howe St.
No injuries or road closures were reported.