Detectives are searching for a man who wore a mask and was armed with a firearm when he held up residents in their Dudley Park home at the weekend.
The man entered the Wanjeep Street house about 11.20pm on Saturday and demanded money from the occupants.
He was given cash before he fled on foot.
Mandurah detectives have called for the public's help to find the offender.
He is fair skinned, 165-172cm tall, with a medium to stocky build and short brown hair.
Anyone with information about the incident is asked to call Crime Stoppers on 1800 333 000.