At about 1.20am, Fire and Rescue NSW Station 392 Muswellbrook and Fire and Rescue NSW Station 208 Aberdeen responded to the blaze.

On arrival, the crews found smoke billowing from a disused building, comprising of two compartments.

The firefighters gained entry into the premises and discovered a working fire in the roof cavity.

While the fire was extinguished, the building suffered moderate damage.

Investigations are now taking place into the cause of the incident.

“A crime scene has been set up,” Hunter Valley Police District Acting Inspector Ryan Froml said.

“And, the forensic team will determine the origins [of the fire].

“At this stage, we’re treating it as suspicious.”