The bodies of four children and three adults were discovered in the town of Osmington, 280km south of state capital Perth, reports the BBC.
The cause of death is not ascertained but the police have confirmed that two firearms were found at the scene.
"The bodies of two adults were located outside (and) five bodies were located inside a building on the rural property," said WA Police Commissioner Chris Dawson.
If confirmed to be a mass shooting, it would be Australia's worst since a massacre in Tasmania claimed the lives of 35 people in 1996.
Dawson said the police would launch a large-scale investigation, and were contacting relatives of the victims.
Osmington is a sparsely populated town about 20km from Margaret River, a popular tourism area.
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