He said the "male person" who had called the police had some connection to the property.
Commissioner Dawson described the event as a "horrific incident".
Mr Dawson said: 'The reason police went to the property was from a telephone call which we've recorded'.
Two adults were found outside the house and the other five bodies were discovered inside.More news: WB rejects reports Nawaz laundered money to India
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None of the victims have been identified yet.
Cynda Miles is well known in the Margaret River area for her involvement in a number of local community projects. Two adults lay on the outside, and another adult and four children inside the building. Dawson said those killed are believed to have been residents at the property. "I worked in the media for 30 years and you report on these things from time to time, but you don't expect them to happen just across the road", he said.
Police say they are unable to share any other details and are now looking for next of kin and friends to notify. I'm not in a position to release any further details about the identities of the deceased.
The homicide squad is assisting police in investigating, and Dawson confirmed there were now no concerns for the safety of the wider public.
Osmington consists only of a handful of roads, farms, vineyards, holiday chalets and horse studs.