Detectives believe the girl went to bed normally around 8 p.m. on Boxing Day before a man broke into Ellenbrook’s family home in the northeastern part of town and assaulted the girl.
He left with the girl’s pajama store and her parents weren’t aware of it until the girl told her father the next day, police said.
Police Inspector Hamish McKenzie said the girl was examined at Perth Children’s Hospital and was with her family following the “totally traumatic” incident.
“The concern of the WA Police is that we have an unknown offender at this point who has committed a heinous sexual assault on a seven year old child,” he said.
Dozens of detectives went door-to-door on Tuesday as the mounted unit scoured the nearby shrub for clues and drones watched from above.
Police revealed that several known offenders living in the area were being interviewed by the sex offender management team.
The only description of the offender that the police could establish is that he was a male.
Detectives are treating the case as a top priority.
Investigators want to speak to anyone who has seen the pajama top, a missing mosquito net from the house, or who has a dashcam or CCTV in the area, especially The Broadway, Tonkin Highway or The Promenade.
Anyone with information can contact Crime Stoppers at 1-800-333,000.