The teen accused of murdering his mother and her two children had sought treatment at a Perth hospital just hours before the deaths.

Teancum Vernon Petersen-Crofts, 19, had gone to the St John of God Midland public hospital emergency department in a "psychotic state" on Saturday night.

But he had been turned away after police became involved, WAToday reported, citing sources.

Petersen-Crofts is charged with murdering Michelle Petersen, her daughter Bella, 15, and son Rua, 8, on Sunday in their Perth home.

WAToday reported that Petersen-Crofts had been behaving erratically on Saturday. However his condition was not considered to be a "medical problem".

Police had become involved as a result of the Saturday night hospital visit but the 19-year-old was returned to his home hours before the deaths were discovered.

The St John of God Midland Public Hospital said it could not comment and referred questions to police. 

Mother Michelle Petersen was killed.

Meanwhile, a friend of victim Michelle Petersen has revealed the harrowing moment she found out about the triple murder, and the family's tragic past.

Toni Ata, known as "aunty", took to social media to express her sorrow following the "horrific" killing of mother-of-five Michelle Petersen, 48, her daughter Bella, 15, and her son Rua, 8.

"I just found out the most devastating news that my friend and her two babies were killed last night … And I'm so, so devastated," Ata said through tears during her live video.

She had already heard about the Ellenbrook murders, but a message from one of Petersen's surviving daughters asking for a phone number confirmed her worst fears.

"I sent it back with a message saying, 'Hey gorgeous, here's the number, hope you're well'," Ata said.

"To which she replied: 'Not really Aunty. Mum & Rua & Bella were killed this morning'."

Ata said she was shocked to the core and immediately started grieving.

"Chills ran through me and the tears came and came and came and came," she said.

"I sit here grieving for the whole family … it's just terrible … such a tragedy."

 

Bella Petersen, 15, who was killed.

She said Petersen had a troubled life, but was doing "so awesome" last time she saw her.

"Michelle had such a hard life," Ata said.

"She lost two babies years ago, when they were babies too: sudden infant death syndrome. "And she was an alcoholic for a really long time.

"I always thought if I lost two babies, I'd be an alcoholic too. Last time I saw her she had been sober for ages."

Rua, 8, was killed along with his mother and sister.

Ata said Petersen's son Rua was the light of her life.

The accused, Teancum Vernon Petersen-Crofts, is Petersen's son, according to Ata.

The 19-year-old teenager was sent to a secure psychiatric facility after fronting court charged with murdering a mother and her two children at a house in a Perth suburb.

Rua and his mother Michelle were found dead inside the home and Bella was found critically injured in the backyard but died on the way to hospital, police said.

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

"It is a very complex and, I would say, horrific crime scene."

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

 

Rua, 8, was killed along with his mother and sister.

It is understood the accused lived with the victims, but his relationship to them remains unclear.

"[Police] located two deceased people inside the home and a further victim in the rear yard," Assistant Commissioner Paul Steel said.

"An ambulance was called in relation to the critically injured victim in the rear yard who was conveyed to Royal Perth Hospital and, tragically, she died en route."

Assistant Commissioner Steel said it could take days for the scene to be processed.

"The home, along with the convenience store, is now a crime scene," Assistant Commissioner Steel said.

"This is a terrible tragedy for the family involved, for their extended family and for those first responders who were forced to attend what was a horrific crime scene.

"I can assure the community that WA Police are applying all resources that we can to get to the bottom of what's happened here."

He said the crime was sure to send shockwaves through the Western Australia community.

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 found a large pair of scissors about 30 metres from the house, the ABC reported.

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, 8 - were found dead at their farm at Osmington, near Margaret River.