Homicide squad detectives are trying to piece together what they have described as the "horrific" killing of a mother and two children at a home in Perth's north-eastern suburbs.

The triple homicide in Ellenbrook was confirmed by police as detectives questioned a 19-year-old man who lived at the house with the three victims and is now in custody.

Officers went to the property on Brixton Crescent in the early hours of Sunday morning after they were initially called to a convenience store on The Broadway at about 1:30am.

"They located two deceased people inside the home and a further victim in the rear yard," WA Police 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."

Police forensic officers were among those investigating what happened at the property. ( ABC News: Robert Koenig-Luck )

Police have not yet revealed the ages of any of the victims but have confirmed they are believed to be a mother, a boy and a girl.

It is believed the woman and her son were the pair found dead inside the house, while the woman's daughter died on the way to hospital.

The street and the convenience store have been cordoned off and a large police presence remained at the scene throughout Sunday.

Police set up a mobile forward command vehicle at the home and forensic officers were among those at the site.

'Terrible tragedy for family': police

Police said the man was helping them with their inquiries, but were reluctant to release more details about his relationship to the three victims before they could contact their next of kin.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 2 minutes 9 seconds 2 m 9 s Three people killed in what WA police are calling a 'complex homicide'

"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," Assistant Commissioner Steel said.

"It's a crime that I know will send shockwaves throughout the community of Western Australia.

"A senior homicide squad investigating officer said to me it is potentially the most horrific crime scene that he has ever been called to."

Assistant Commissioner Steel said police were doing all they could to "get to the bottom of what's happened here".

"My heartfelt condolences go out to all of those who are affected by this tragic incident," he said.

Man asked for police at convenience store

Officers were called to the convenience store after the 19-year-old man entered the premises and requested police attend.

A 7-Eleven store near the house was also taped off by police. ( ABC News: Robert Koenig-Luck )

As investigations continued into the morning on Sunday, officers photographed a large pair of scissors lying in a verge garden bed about 30 metres from the house before placing them in an evidence container.

Police have appealed for anyone with information about what happened to come forward.

"We are continuing to commit as many resources as are required to investigate this horrific crime," Assistant Commissioner Steele said.

"But in the interim, I do ask that any members of the public who think they may have information which may assist police in relation to this investigation to contact Crime Stoppers on 1800 333 000."

Yelling and screaming heard from house

Neighbours said they had seen police using torches to look into surrounding houses at about 2:00am.

Matthew Schreck, who lives around the corner from the house, said he had been woken by a commotion coming from the home about an hour earlier.

Matthew Schreck said police arrived on the scene soon after he heard noises from the house. ( ABC News: Nicolas Perpitch )

"I just heard a lot of screaming and yelling and a bit of fighting, and the cops flew past here and headed up this way," Mr Shreck said.

"It was real loud yelling and screaming. It sounded pretty bad. It sounded like a man."

Mr Schreck said it was unusual for the area and police responded quickly.

A police spokeswoman said officers were still trying to determine what happened.

"There are police officers there, they are still assessing the scene," she said.