UPDATE: Police have now confirmed the three dead people are a mother and her two children.
They have not confirmed the ages of the male and female child.
EARLIER: Police have revealed they are investigating the deaths of three people at a house in Ellenbrook overnight.
Officers were called to a convenience store on The Broadway in Ellenbrook after a man had appeared there asking for police at 1.30am.
After police attended the convenience store, they went to a nearby house.
When they got to the house, they found a man and a woman dead inside and a female in the back yard with critical head injuries.
The woman died after being taken to Royal Perth Hospital by ambulance.
A 19-year-old man who lives at the house is in custody and helping police with their inquiries.
Homicide Squad detectives and forensic officers remain at the scene of the house.
Anyone with information on the deaths should contact Crime Stoppers on 1800 333 000 or make a report online.