At the scene they found the bodies of seven people.
One woman was located dead inside the main house, while another woman and four children were discovered inside a converted shed. Two firearms were found.
"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", Dawson said.
The police chaplain has been supporting officers through what Commissioner Dawson described as one of the most confronting police have had to deal with.
They included Ms Katrina Miles, 35, and her four children - three boys and a girl, aged eight to 13.
Not only because an entire family is involved - but for the four young children; their lives have been violently snatched away before they've barely begun.
"When Australia had a mass killing-I think it was in Tasmania-about 25 years ago, it was just so shocking the entire country said, 'Well, we're going to completely change our gun laws, ' and they did", he told comedian Marc Maron during a podcast taping.
Facebook posts by Katrina and Cynda Miles, quoted by local media, referred to the children as all being autistic and homeschooled.
WA Premier Mark McGowan has offered his sympathies to the family of the victims, describing the incident as "appalling, bad and terrible".
Police were attempting to make contact with victims' relatives, Dawson said.
There's an eerie feeling about the tourist town of Margaret River.
Homicide detectives are now assisting police, he said, adding that there was no danger to the public.
A man called 911 at 5:15 a.m.to report the murders, officials said, but would not identify who the caller was.
The case is particularly unusual given Australia's strict gun laws, which are often pointed to as examples for other countries struggling to prevent mass shootings. "We are stunned and still trying to understand how this could happen", they said in a statement.
The generally accepted definition of a mass shooting - four deaths excluding the shooter in a single event - has been met only once in Australia since then.
Red Cross counselling services remain available to the public at the Margaret River Community Centre, where friends have also been placing floral tributes. Mrs Miles has been a stalwart in the sustainability community, volunteering with the local Transition Network and leading recycling initiatives including Cynda's Soft Salvage.