The victims – three generations of the same family – were discovered in the rural village of Osmington.
They have now been officially named as Katrina Miles, her four children, aged eight to 13, and her parents, Peter and Cynda Miles.
Three firearms found at the property belonged to Mr Miles, officers have revealed.
“I wish to strongly emphasise that police do not believe any other person is involved in these crimes. Police are not searching for any other suspects,” said commissioner Chris Dawson.
He added that officers had received a phone call from a man at the property alerting them to the incident on Friday morning.
On investigation, the body of a woman was found in the house at the property, while the bodies of a woman and four children were found in a nearby converted shed where Ms Miles lived with her three sons and daughter. A seventh person was found dead outside.
It is the country’s deadliest mass shooting since a massacre in Port Arthur, Tasmania, claimed the lives of 35 people in 1996.
Relatives of the Miles family have said they are “stunned” by what has happened.
In a statement issued through the police, the unnamed family members said they were “trying to understand how this could happen”.
They continued by asking “the community refrain from speculating on the circumstances around this tragic incident”.
Osmington is a tiny rural village 20km from Margaret River, a popular tourist area.
Ms Miles, 35, and her children are reported to have moved into the property, owned by her parents Peter, 61, and Cynda, 58, three years ago.
She had homeschooled the youngsters, and was described by friends as a devoted mother.
One neighbour, Felicity Haynes, described them as “lovely people”.
“They were a very socially-aware family – doing their best to create a safe community – and that is why it is so shocking to think that could be destroyed so quickly,” she told the Australian Broadcasting Corporation.
Pamela Townshend, president of the Shire of Augusta-Margaret River, told Fairfax Media: “It’s sending shockwaves through the whole community – we’re all linked in one way or another, every family.”
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