Two firearms were found on the property.
He went on: "Upon arrival, police located seven persons deceased". They found seven people dead on the property, including four children.
West Australian Police Commissioner Chris Dawson said officers were called to the property in the Margaret River area at about 5.15am (WA time) today, and described the deaths as a "significant tragedy".
There was no threat to public safety, he said.
Two firearms were recovered outside the property.
The Western Australia police commissioner Chris Dawson confirmed that four children and three adults have been killed in a suspected mass shooting in the southwest of the state, 9 News reported. The bodies of two additional adults were located a short distance away.
Police are expected to be at the property for several days while the investigation into this horrific tragedy takes place.
Other members of the families are in the process of being located.
"This devastating tragedy will no doubt have a lasting impact on the families concerned, the whole community and, in particular, the local communities in our south-west", Commissioner Dawson said.
Specialist units from Perth, including the Homicide Squad and Forensic Crime Scene Unit, have deployed to the scene.