Seven people have been found dead with gunshot wounds at a property near the Australian tourist town of Margaret River, police said.
The bodies of three adults and four children were found at a property in the village of Osmington along with two guns, Western Australia state police commissioner Chris Dawson said.
Police were called to the house at around 5.15am (local time) on Friday and officers said they had no concerns about wider public safety, Mr Dawson added.
"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 southwest," he said.
Police were attempting to make contact with the victims' relatives, Mr Dawson said.
The commissioner would not comment on the possibility of murder-suicide.
This could be the worst mass shooting in Australia since a lone gunman killed 35 in Tasmania in 1996, prompting the nation to introduce tough gun controls.
Australia's gun laws are widely acclaimed as a success, with supporters including former US president Barack Obama saying Australia has not had a single mass shooting since they were implemented.
The generally accepted definition of a mass shooting - four deaths excluding the attacker in a single event - has been met only once in the country since then. In 2014, a farmer shot his wife and three children before killing himself.
Farmers are allowed to own guns under Australian law due to a legitimate need to use them to kill feral pests and predators or sick or injured livestock.
But automatic and semi-automatic rifles and shotguns are banned from public ownership.
Osmington is a collection of a few streets, vacation accommodation and vineyards supplying the premium winemaking district known as Margaret River.
Samantha Lee, chair of the Gun Control Australia lobby group, said rural areas were over-represented in Australian gun deaths, including suicides.
"Regional and rural areas are particularly vulnerable to these sorts of tragedies, because of the combination of isolation, sometimes mental or financial hardship and easy access to firearms," Ms Lee said in a statement.
"Although the details of this tragedy are yet to come to light, Australia has a tragic history of a higher rate of gun deaths in rural areas," she added.