"Police have located the bodies of a number of deceased people including women and children", he said but would not say how they died or if any weapons were found.
Cops went to the property in the northern suburb of Perth after a man visited a regional police station and told them about the scene.
The man now assisting police remains in custody, no charges have been laid at this time.
Police refused to say what the man told them - or how he was known to the victims.
Homicide detectives and specialist forensic police will continue to comb the Coode Street home for evidence.
"It is a tragic thing when incidents like this occur".
"There are no ongoing concerns for public safety as a result of this incident", they said.
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.
When police entered, they discovered the bodies of two women and three young children.
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Assistant Commissioner Paul Steel of the Metropolitan Region said it was a tragedy and the scene was being forensically examined.
"It's horrid to think that in such a handsome area, that's calm and so quiet, that something so tragic has taken place", she said.
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'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.
No charges have been laid.
Mr Steel stressed that this was very early in the investigation so he could not provide many details.
"Up to five" bodies at the home in suburban Perth are believed to have been a mother, her children and their grandmother, news.com.au reported.
The children's grandfather, Peter Miles, was also found dead from a gunshot and three firearms belonging to him were found on the property.