Thiess, in alliance with Parsons Brinkerhoff (PB) and Hyder Consulting, is building the 13.3km eastern section of the four-lane dual expressway.
Police and fire fighters were called to the Hunter Expressway construction site in Buchanan where two excavators were found to be alight. Upon further investigation, police found a third excavator which had been destroyed by fire.
The fires took several hours to extinguish as diesel fuelled the fires. Other heavy vehicles parked at the construction site had also sustained damage.
The fires were deliberately lit and police are appealing for anyone who saw anything suspicious in the vicinity of the construction site to come forward.