The shooting is believed to have been a murder-suicide but this is yet to be confirmed.
"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".
"The loss of any life is tragic but four children and three adults, this is a significant tragedy", he said, adding that police and other specialists would be at the scene for several days at least.
Authorities would not say how the victims died, but confirmed that two firearms were found at the scene.
Commissioner Dawson also urged anyone with any information about the Osmington tragedy to contact police as they begin to comb through evidence.
The commissioner would not speculate on a motive but said that, at this point, police had no concerns about wider public safety.
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The victims were most likely residents of the property, authorities said.
Dawson said police were still trying to locate other family members and did not release the identities of any of the deceased.
Mr Dawson said: 'The reason police went to the property was from a telephone call which we've recorded'.
A police chaplain is at the scene to support assisting officers.
Commissioner Dawson said officers were not looking for any suspects.
It is understood detectives from the major crime squad are travelling to Margaret River from Perth this morning.
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Federal MP for Vasse Libby Mettam, whose seat takes in Margaret River, said the deaths had already sent "significant shockwaves" through the town.
Osmington only consists of a handful of roads, farms, vineyards, holiday chalets and horse studs.
Osmington is a sparsely populated town about 20km from Margaret River, a popular tourism area.
Premier Mark McGowan said the incident was shocking.
"This is a much-loved family, a big part of the Margaret River community", she told ABC News.
Local MP Libby Mettam said her heart went out to everyone affected by the tragedy.
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