Police are investigating an armed hold-up at a petrol station on Mirrabooka Avenue in Alexander Heights overnight.
The male offender walked through the unlocked front door and threatened the female worker with a garden tool, before escaping with the money.
He is described as approximately 170cm tall, and he had his head and face concealed.
Police have not yet found the offender, after conducting a wide search of the area using canine and response units.
Anyone with information is asked to call Crime Stoppers on 1800 333 000.