"This is early and we've not yet fully examined the scene. We understand that there are adults and children involved," Western Australia Police Assistant Commissioner Paul Steel told a news conference.
Steel said up to five bodies had been found at the scene when police arrived at noon. Investigators were still there.
"I can say there was at least one woman but I can't say more than that I'm afraid. The scene has not yet been forensically examined so the exact number of adults and children is actually unknown to me," Steel said.
He did not say how the people were killed or if any weapons were found.
A man in his 20s was helping police with their inquiries, he said, adding there was no danger to the public.
Violent crime with multiple victims is rare in Australia but this was the second major incident in Western Australia this year.
Seven people were shot and killed in the state in May.
Gun crime in Australia fell sharply after strict controls were introduced in response to a mass shooting in 1996, when a lone gunman killed 35 people in Tasmania.