Alleged Perth Killer's Bizarre Rant In Court

It is understood the accused is related to the three victims.

A 19-year-old charged over an horrific triple murder in Perth has ranted to a Magistrate during his first court appearance.

Teancum Vernon Petersen-Crofts was handcuffed and flanked by three security guards during Monday morning's brief appearance in the Midland Magistrates Court.

During the proceedings, he referred to a man named "Griff", saying "he's a criminal, he did it."

Petersen-Crofts is charged with the murder of his eight-year-old brother Rua, his 15-year-old sister Bella and his 48-year-old mother Michelle.

Michelle Petersen and he children Bella and Rua were killed on Sunday. Images: Facebook

Their bodies were found at an Ellenbrook home in the early hours of Sunday morning.

It is understood the accused was son to Michelle and siblings to Bella and Rua.

In court, Magistrate Greg Smith told Petersen-Crofts "you don't have to say anything"

But the alleged killer continued, saying "I need to, because Mama is with me."

People are seen outside a house where a mother and two children were killed in Ellenbrook. Image: AAP

He went on to say he had been living with 'Griff' for two months, and said he "had to leave because I was that scared he was going to kill me. Mama told me to save myself."

Petersen-Crofts will remain in custody at a secure psychiatric centre until his next appearance on the 23rd of July.

Assistant Commissioner Paul Steel told reporters on Sunday that the crime scene is complex and one of the most gruesome he had been called to.

“A senior homicide detective squad officer said to me that it is potentially the most horrific crime scene that he has ever been called to,” 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 ... my heartfelt condolences go out to all of those who have been affected by this tragic incident."