Police are investigating an aggravated robbery at the Caltex service station on Bridge Road, Para Hills. 

At about 4.40am, a man wearing a ski mask entered the service station when the doors were opened for a customer. 

The offender was allegedly armed with a knife and stole two packets of cigarettes and a small amount of cash. 

He was last seen on foot towards the carpark of the Somerset Hotel. 

The offender is described as Caucasian, fair skin and with some marking on his left wrist. 

He was wearing black shoes with a white stripe, black tracksuit pants, grey hooded top and a white t-shirt. 

He wore a full face black ski mask with white hoops around the eyes and mouth openings.