A LOUD explosion could be heard moments before a Gympie house went up in flames on Sunday night.

Within seconds an Old-Queenslander property on Blake Street was fully engulfed just after 9.40pm.

Four fire and rescue units, two police and an ambulance unit rushed to the scene.

Gympie Station officer Cameron Nicol said nobody was home when the fire started.

"We responded earlier tonight to a structure fire. On arrival, we found a house well involved. Crews have conducted internal searches and established nobody was home and then they've gone about extinguishing the fire," Officer Nicol said.

He said officers will be investigating how the fire started. 

"At this stage we haven't established a cause," he said.

"The house has sustained large amounts of structural damage."

It took around 10-15 minutes for officers to extinguish the blaze.

Neighbours living on Watt street reported hearing an explosion, similar to a gas bottle exploding before seeing large flames engulf the property.

"We heard an explosion and didn't think much of it until we heard the fire brigades rush past," one witness said.