The owners of a Cowaramup dairy destroyed by a suspicious fire overnight say the damage bill tops $5 million.
The blaze started at the Margaret River Dairy Company at Fonti Farm on Bussell Highway about 2.50am and took more than an hour to bring under control.
About 30 volunteer firefighters from Metricup, Cowaramup, Yallingup, Dunsborough and Busselton attended.
A Department of Fire and Emergency Services spokesman said crews were still at the scene as the blaze was yet to be fully extinguished.
Fire investigation officers and police arson squad detectives are en route.
Those with information are asked to contact Crime Stoppers on 1800 333 000.