Once inside the two-bedroom suburban brick-and-tile house, police discovered the bodies of Mara Harvey, her three daughters - two-year-old twins and a three-and-a-half year old - and the girls' grandmother Beverley Quinn.
"The man now assisting police remains in custody".
Specialist forensic police are expected to continue a meticulous examination of the crime scene on Monday.
Western Australia assistant police commissioner Paul Steel said: "This is early and we've not yet fully examined the scene".
What the man told police is not yet known but it prompted officers to descend on the Bedford home just before midday.
Maria Manfredini, who lives at the property behind where the incident occurred, said she was shocked by the tragedy.
The horrific discovery came in the Perth suburb of Bedford, in Western Australia.
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Police were on Sunday night yet to confirm the identities of the deceased but it is believed they are a mother, her three children and another older female relative.
POLICE have discovered up to five bodies, including those of children, following a "tragic incident" at a house in Western Australia (WA).
There are no ongoing concerns for public safety.
Neighbors said they often saw children playing in the front yard of the home.
No charges have been laid.
"I can assure people that the WA Police Force are conducting a thorough and proper investigation to determine what events led to this tragic matter".
"It's just so sad, they've got primary school aged kids and I thought it just couldn't be", she said.
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"They were a ideal family. this is a attractive neighbourhood, it's quiet", Alfie said.
"This is a tragic event and it will no doubt have an impact on not only the family and friends of the deceased, but on the whole of the community, those first responders who are faced with attending a scene with deceased people."
Three firearms were located at the scene belonging to grandfather Peter Miles.
It has taken everyone by surprise, we are all in complete shock.
When officers went to the house, they found Michelle Petersen and her son, Rua, dead, and her daughter Bella in a critical condition.
In July a man allegedly killed his mother and two siblings.
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