A 19-year-old man has been charged with the murders of an eight-year-old boy, a 15-year-old girl and their mother at a house in a Perth suburb.

WA Police say the man will appear before Midland Magistrates Court on Monday to face three murder charges.

Police on Monday released the ages of the family. The mother was 48.

The boy and his mother were found dead inside the home and the teenage girl was found critically injured in the backyard but died on the way to hospital, police say.

Assistant Commissioner Paul Steel told reporters in Perth on Sunday, a senior homicide squad investigating officer had said it was “potentially the most horrific crime scene he has ever been called to”.

The area was quickly cordoned off after the horrific discovery. Photo: AAP

“It is a very complex and I would say horrific crime scene.”

The man, who also lived at the property, is expected to appear in Midland Magistrates Court on Monday.

His relationship with the woman and children has not yet been confirmed.

Police were initially called about 1.30am on Sunday to a convenience store on The Broadway before they went to the home.

Mr Steel said it could take days for the scene to be processed and he had limited details to release.

“It is a very complex and I would say horrific crime scene. It will take us some time to process, and that could be days,” he said.

Police were shining torches in neighbouring Ellenbrook homes in the early hours on Sunday. Photo: Twitter/Nicolas Perpitch/ABC

A man who lives in a neighbouring street earlier told The West Australian he was woken by screaming and yelling about 1am.

“It was real loud screaming and yelling. It sounded pretty heavy,” Matthew Schreck said.

Police were very tight-lipped about the incident before the press conference.

“It’s a crime that I know will send shockwaves throughout the community of Western Australia,” Mr Steel said.

Forensic work at the home could take several days, police said. Photo: AAP

“I can assure members of the the community that WA Police are applying all the resources that we can to get to the bottom of what has happened here.”

The deaths come just two months after Peter Miles, 61, his 58-year-old wife Cynda, their daughter Katrina, 35, and her four children – daughter Taye, 13, and sons Rylan, 12, Arye, 10, and Kadyn, eight – were found dead at their farm at Osmington, near Margaret River.