Emergency services were called to a blaze which broke out at the Monadelphous building in Narromine over the weekend.
About 1pm on August 10, emergency services including the Narromine and Trangie Fire and Rescue NSW, the Rural Fire Service and the Hazmat team from Dubbo attended the abandoned factory.
It is believed a small lit fire escaped igniting two large bales of hay located on the front wall of the northern end of the building.
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According to Narromine Fire and Rescue NSW captain commander Ewen Jones it took crews three hours to take the blaze under control.
"It was difficult to put out," he said.
"Our thermal imaging cameras didn't pick up the hot spots because hay insulates it."
Captain Jones said the owner of the building organised a backhoe to spread the hay over the cement so crews could extinguish it.
It wasn't a big fire, it was a difficult one.Narromine Fire and Rescue captain commander Ewen Jones
"It wasn't a big fire it was a difficult one," he said.
The fire captain said the power had already been disconnected, and the building was reportedly scheduled for demolition.
The building was originally built by McPherson and Cluness opened it in the 1960s and used to build farm machinery.
The mining company Monadelphous took over the building in the 1990s, but since moving out the building has been unoccupied.
Dubbo detectives attended the scene and are investigating the fire.
Anyone with any information is urged to come forward and contact Crime Stoppers on 1800 333 000.
Captain Jones is also reminding people to call triple zero (000) during a fire.
"Some people are phoning a friend or calling someone that works on the brigade ... it just delays the response and it's happening a lot," he said.
"It doesn't cost anything ... and it's much quicker getting us on the road when you call triple zero."