Police investigating suspicious fire at Auckland's Birkenhead College

ZIZI SPARKS/Stuff

Firefighters were called to a fire at Birkenhead College on Auckland's North Shore on Wednesday night. (file photo)

A suspicious fire has damaged a toilet block at an Auckland secondary school.

Fire and Emergency NZ was alerted to the fire at Birkenhead College on the North Shore about 7.50pm on Wednesday.

Three fire trucks responded to the incident, fire communications northern region shift manger Daniel Nicholson.

The toilet block was 22 metres by eight metres in size, he said. The fire was eventually put out about 8.30pm.

"We have a fire investigator at the scene," Nicholson said.

A police spokeswoman said police were investigating a "small fire" in a toilet block at the school.

Police were called to the school at 8.11pm, she said.

"No one was injured during the fire and it is being treated as suspicious."