A CAR filled with drums of petrol was driven into the CFMEU's NSW head office before being set alight.

The union believes its office was the target of last night's firebomb attack on a western Sydney office block.

Construction, Forestry, Mining and Energy Union (CFMEU) state secretary Andrew Ferguson said a car filled with drums of petrol crashed into the union's Lidcombe office before being set alight.

The ground floor of the Railway St premises was extensively damaged in the fire, while upper levels suffered smoke damage.

No one is believed to have been injured in the blaze although it is understood a church community group had been using the building an hour before the attack.

"This was a very determined attack on our building that caused extreme damage," Mr Ferguson said.

"Our primary concern is for our staff and the community groups who use our offices.

"We are very lucky people weren't killed here."

Mr Ferguson believed the incident may have been linked to a dispute between the union and a contractor.

Police are continuing to investigate the incident, which occurred about 10.30pm (AEST) last night.

Originally published as Union head office hit by car bomb