At 6.30pm last night, a man entered the Shell service station in North Haven carrying a small knife and demanded money from the console operator.
A small amount of cash was given and the man was last seeing running along Pioneer Street North Haven.
The alleged assailant is described as being about 172cm tall (5ft 8in), of thin build. He was wearing dark tracksuit pants, a hoodie and his face was covered with a cloth.
No one was injured during the hold up.
Police attended the scene and searched the area, speaking to a number of people.
Detectives are studying CCTV footage and investigations will continue today.
If you have any information for police, call the Police Help line on 131 444.