A fire in the changing sheds of a North Shore school last night is thought to be suspicious.

Police and Fire and Emergency New Zealand were called to the fire at Birkenhead College at 7.50pm.

Three fire crews put out the fire.

Principal Craig Waller said it damaged doors in the changing sheds, as well as part of the wooden ceiling.

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"The lucky thing was that it is a brick building, no one was hurt, and it is actually a long distance from the rest of the school.

"We are very much waiting to hear back from the police in terms of their investigation," he said.

"If someone has lit the thing then hopefully the police get somewhere with their investigation."

A police spokesperson confirmed they were investigating the fire in the standalone, 22m x 8m structure, which is some way from the rest of the school buildings.