The mother and her two children slain in their Perth home have been identified by a friend who has described their deaths as "terrible" and revealed their tragic family past.

Michelle Petersen, 48, and her daughter Bella, 15, and Rua, 8, were killed in their family home on Brixton Crescent in Ellenbrook, Perth, on Sunday morning.

Teancum Vernon Petersen-Crofts, 19, who is understood to be the son of Michelle and brother of the other victims, has been charged with their murders.

A friend of the family, Toni Ata, posted a live video on social media after the tragedy.

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"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," she said through tears.

LEFT: Michelle Petersen described her son Rua as her 'future NASA boy' in a Facebook post. RIGHT: Rua pictured on his fifth birthday. The 8-year-old was killed over the weekend. Photos / Facebook
LEFT: Michelle Petersen described her son Rua as her 'future NASA boy' in a Facebook post. RIGHT: Rua pictured on his fifth birthday. The 8-year-old was killed over the weekend. Photos / Facebook

Ata said she found out about the murders through one of Petersen's surviving daughters, who sent her a text message "asking for a friend's number".

"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 we're (sic) killed this morning.'"

Ata had already heard about a triple murder having taken place in Ellenbrook so the pieces quickly came together.

"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."

Disturbed police officers said the bloody crime scene was the worst they had seen after making the grim discovery at a house in Ellenbrook, about 30 minutes' drive northeast of the Perth CBD.

As they opened the door, officers saw Rua and Petersen dead inside, while his sister Bella was in the backyard suffering from critical injuries.

An ambulance was called to the scene but the injured teenager died on the way to Royal Perth Hospital. Police were seen taking a pair of scissors from the home yesterday afternoon.

Teancum Vernon Petersen-Crofts has been charged with the murders. Photo / Facebook
Teancum Vernon Petersen-Crofts has been charged with the murders. Photo / Facebook

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".

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

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

Ata said Petersen "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 was the "light of [Ms Petersen's] life", according to Ata.

"She just loved him," she said.

"It's so sad that he's gone too; [he was] such a cute little boy."

Petersen-Crofts will face Stirling Gardens Magistrates Court next week.

Accused's bizarre court outburst

Earlier today, the accused appeared to have a psychotic episode in a Perth court.

"He's a criminal," Petersen-Crofts said of a person who is unknown at this stage, at his appearance before Midland Magistrates Court..

"He went to take me out bush. He did it. He came back from my family."

When Magistrate Greg Smith told him he did not need to speak, Petersen-Crofts kept talking.

"I needed to because mama is with me... I was painting a picture of Jesus and stayed there for two months. He punched me so hard in the head. So hard.

"I left because I was that scared he was going to kill me. Mama told me to save myself."

When court security attempted to calm him, Petersen-Crofts lashed out; "What are you looking at, serial killer? He looks like a serial killer and he's a security guard."

Magistrate Smith said due to concerns regarding Petersen-Crofts' mental health, he would be sent to Franklin Centre Graylands Hospital, until his next court date on July 23.

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.