Steel said up to five bodies had been found at the scene when police arrived at noon.
A man aged in his 20s alerted police on Sunday morning.
Locals said children were often seen playing in the front yard of the home.
'The scene has not yet been forensically examined so the exact number of adults and children is actually unknown to me'.
"The man is now in custody and assisting police with their inquiries". No further information was provided on the victims and the man in custody.
The police also stated that "there are no on-going concerns for public safety as a result of this incident".
"It's just so sad, they've got primary school aged kids and I thought it just couldn't be", she said.
"We've just been on a holiday and come back yesterday to silence in the street", resident Richard Fairbrother, who lived next door to the house said.
'This is a tragic event and will have an impact on family and friends, the community, and the emergency services who first responded to the tragic scene.
Homicide detectives and specialist forensic police will on Monday continue to comb the Coode Street home for evidence and clues to what happened after the shocking discovery on Sunday.
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Mr Steel said it was still unclear how long the bodies had been at the house before they were discovered.
"This is a very close community".
A number of police cars, homicide squad officers and forensic police were at the house, with the road blocked off at the Coode and Beaufort St intersection.
"It's just horrific on every level".
'They just seemed like a normal family, you would never imagine anything like this happening here, ' Mr D'Souza said.
"We're all in complete shock".
WA Police Assistant Commissioner Paul Steel confirmed the tragic news in a press conference at 5pm AEST.
It comes after the shooting deaths of three adults and four children in Osmington in regional WA in May, and the deaths of a mother and her 15-year-old daughter and eight-year-old son at Ellenbrook, in Perth's northeast, in July.