The offender entered the Wanneroo Road outlet as the manager was closing up about 10.30pm on November 5.
He did not produce a weapon, but indicated he had a firearm while demanding cash.
The female staff member fled the store through the front door to seek help from a nearby business.
The man was described as about 180cm to 185cm tall and was wearing a black hooded jumper.
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There was not CCTV footage of the incident.
Anyone with information can contact Crime Stoppers on 1800 333 000.