The bodies of four children and three adults were discovered in the town of Osmington, 280km (170 miles) south of Perth, on Friday.
Authorities would not say how the victims died, but confirmed that two firearms were found at the scene.
There was no ongoing threat to public safety, police said. They would not confirm reports of a murder-suicide.
“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.
“It appears that gunshot wounds are there, but I don’t want to go further than that.”
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.