A large fire that tore through a block of old classrooms in Greymouth is being treated as suspicious

Fire crews received multiple calls to the former Karoro Learning site on Tainui Street, yesterday afternoon.

Forty firefighters from Greymouth, Cobden, Runanga and Brunner were called, with a block of four classrooms well alight on arrival.

Greymouth chief fire officer Lee Swinburn says the building has sustained substantial smoke and water damage and won't be saved.

He says police and a fire investigator will be on site this morning.