Police and detectives have swarmed the 7Eleven fuel station in Mandurah on Thursday.
About 12:15pm, it’s alleged a man entered the Pinjarra Road store with a weapon, covered his face, and demanded cash.
Staff handed over a sum of money and the offender fled the store.
Detectives cordoned off the fuel station for over an hour as they gathered evidence.
The offender, believed to be in his 20s, is described as having a slim-to-medium build and olive skin.
He was wearing a white top and red shorts.
The store has since reopened.
Anyone with information is urged to contact Crime Stoppers on 1800 333 000