The latest blaze is expected to disrupt production at the multi million dollar factory with owner Glenn Scott estimating that more than $4 million damage had been caused.

Production at the plant was closed down when fire caused millions of dollars worth of damage to the main part of the factory in September 2006.

A passerby raised the alarm about the latest fire at 3 36am on Sunday morning.

Cowra Fire Brigade captain Derek Brown said two Cowra units were quickly on the scene but it wasn't until just after 5am that they were confident they had contained the fire to the one building at the Sarajane complex.

The building destroyed in the latest fire is flanked by two other sections of the factory.

Mr Brown said upon arrival at the scene in, William Street, the Cowra Fire Brigade called for two further stations to assist.

This assistance came from Canowindra and Grenfell with brigades from Bathurst and Young also called in later in the morning.

Police, ambulance and Country Energy staff also assisted in the operation.

Detectives and members of the Police Forensics Group were on the scene yesterday, attempting to determine the cause of the latest fire which is believed to have started at the rear of the factory's canteen.

The building also contained spray booths and machining areas. Police investigations into the fire are continuing.