About 2am on Sunday, Muswellbrook Security and Muswellbrook Police were alerted of the emergency by smoke alarms installed in the building.

The blaze was in the main servery area, which was consequently destroyed and there was extensive smoke damage to the kitchen and dining room.

An insurance assessment on Monday indicated the cost of the fire would be between $50,000 and $100,000.

RSL Club chief executive officer Daryl Egan said when he arrived at the scene security and police had used five fire extinguishers in attempts to douse the fire, but thankfully Muswellbrook and Aberdeen Fire Brigades arrived soon after.

Mr Egan said the whole kitchen had been refurbished only two years ago.

He said the exact amount of damage had yet to be determined but all the ceilings and tiles would need to be replaced, as well as new equipment and paint work to the kitchen and main dining area.

"The club is trading as normal but the bistro is naturally out of action," Mr Egan said.

"We hope to be back to business in a couple of weeks."