Fire crews are responding to a fire they are treating as suspicious at the changing sheds of a North Shore school.

Polices and Fire and Emergency New Zealand responded to a fire at Birkenhead College at 7.50pm.

A fire investigator is currently on the way and the fire in a changing room is being treated as suspicious.

"It seems to stand alone, away from the school at the sports field," a spokesperson said.

Three fire crews were working to extinguish the fire from the structure measuring 22m x 8m.