The fire at Johnson Asahi began on Monday at 3am and has burnt hay, forklifts, loaders and an x-ray machine. Fire controllers expected the smoke to affect Horsham residents until at least tomorrow.

The police helicopter and crime scene investigators will visit the Plumpton Road business on Friday to determine the cause of the fire.

Detective Senior Constable Dave Ellis, of Horsham's Criminal Investigation Unit, said Victorian forensic scientists and arson chemists were at the scene yesterday.

"We're treating it as suspicious at the moment and will be making forensic examinations and other inquiries this week," he said.

"The air wing will evaluate the scene by undertaking digital imaging and video footage for us to examine."

Detective Sen Constable Ellis said police were appealing for witnesses.

"Anyone who contacted fire or police to inform them of the fire or who may have been in the area around 3am on Monday can contact us at CIU or if they want to be anonymous they can contact Crime Stoppers," he said.

Fire incident controller Ian Walter said firefighters would allow the hay to burn itself out.

"We are using excavators to pull hay out, so it will burn quicker," he said.

Region 17 operations officer Richard Bourke said at least 35 firefighters from Horsham, Wonwondah, Noradjuha and Ballarat City brigades attended the fire.

Mr Bourke said Ballarat City provided a large telescopic platform, truck, pumper and refilling truck and helped with breathing apparatus for firefighters.

"We contained the fire to a compartment of the shed at about 2pm on Monday," he said.

"We were able to prevent it from getting into more machinery in the shed."

Mr Bourke said Green Lake and Vectis brigades attended the fire later on Monday.

"About 15 firefighters worked through the night shift on Monday night and then another 15 on Tuesday," he said.

Mr Bourke said the fire would continue to be monitored last night.

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