Four children and three adults have been shot dead in a home near the West Australian town of Margaret River, in what is the nation's worst mass shooting since the 1996 Port Arthur massacre.

Key points: Police were tipped-off by a man connected to the property

Police were tipped-off by a man connected to the property Two adults were found outside the home, five bodies inside it, including four children

Two adults were found outside the home, five bodies inside it, including four children It is the worst mass shooting in Australia since Port Arthur in 1996

Police are treating the incident as a murder-suicide and have confirmed they are not looking for any suspects.

A friend of the family has confirmed to the ABC that Katrina Miles and her four children were among the seven victims.

The friend confirmed that Katrina's parents, whose names are listed on the property title as Peter and Cynda Miles, were also killed in the incident.

Katrina Miles and her four children were killed on the family property in Osmington, near Margaret River in WA's South West. (Facebook: Katrina Miles)

WA Police Commissioner Chris Dawson said all seven victims appeared to be residents of the property, which consists of an established house and a shed that had been converted to living quarters.

Two adults were found outside and the other five bodies were discovered inside the converted shed.

Cynda Miles was found dead on her property. (Facebook: Cynda Miles)

Police at the scene confirmed to the ABC that the family's next of kin had been notified, but they were yet to formally confirm the identities of the victims.

The mass shooting has the most fatalities since the Port Arthur massacre, in which 35 people died and another 23 were seriously injured.

Phone tip-off to police

Commissioner Dawson said officers were called to the rural property in Osmington, about 20 kilometres north-east of Margaret River, about 5:15am on Friday.

They found firearms at the crime scene.

"It appears that gunshot wounds are there, but I don't want to go further than that as two firearms have been located," Commissioner Dawson said.

Sorry, this video has expired Seven people found dead in Margaret River house

Detectives from the Homicide Squad and Forensic Crime Scene Unit have travelled to the scene as part of the investigation.

If you or anyone you know needs help: Lifeline on 13 11 14

Lifeline on 13 11 14 Suicide Call Back Service on 1300 659 467

Suicide Call Back Service on 1300 659 467 Beyond Blue on 1300 22 46 36

Beyond Blue on 1300 22 46 36 Headspace on 1800 650 890

Commissioner Dawson announced the discovery at a media conference in Perth, describing the deaths as a "horrific incident".

He said a "male person" connected to the property had phoned police just after 5:00am, and officers arrived at the scene shortly afterwards.

"The loss of any life is tragic, but four children and three adults, this is a significant tragedy," he said.

"This will be a very large-scale and detailed investigation."

Commissioner Dawson said the phone call to police had been recorded but he would not reveal any more details about it.

Police say they expect to remain at the property for "an extended period of time". (ABC News)

Officers went to the home after a man connected to the property called police. (ABC News: James Carmody)

Commissioner Dawson said police were not looking for a suspect.

"We don't have any information to raise further public concern about that element," he said.

Worst mass shooting in 20 years

Prior to this incident, the worst mass shooting — defined as a shooting involving five or more victims — since Port Arthur involved a man killing himself and four members of his family on a rural New South Wales property in Lockhart in 2014.

Australia's worst modern mass shootings Date Location and state Total deaths (including shooter) May 11, 2018 Osmington, WA 7 September 9, 2014 Lockhart, NSW 5 April 28, 1996 Port Arthur, Tas 35 January 25, 1996 Hillcrest, Qld 7 March 31, 1993 Cangai, NSW 6 October 27, 1992 Terrigal, NSW 6 August 17, 1991 Strathfield, NSW 7 August 30, 1990 Surry Hills, NSW 5 September 25, 1988 Oenpelli, NT 6 December 8, 1987 Queen Street, Vic 9 October 10, 1987 Canley Vale, NSW 6 August 9, 1987 Hoddle Street, Vic 7 June 19, 1987 Top End, NT/WA 6 June 1, 1984 Wahroonga, NSW 6 September 24, 1981 Campsie, NSW 6 Source: JAMA *Mass shooting defined as having five or more victims

Mass shooting expert Glynn Greensmith said these incidents have become incredibly rare in Australia.

"We have been lucky that all forms of gun-related deaths have reduced dramatically since we took that step of having less guns [following the gun bans in the wake of Port Arthur]," said Mr Greensmith, who is studying a PhD in mass shootings at Edith Cowan University.

"But there are as many guns now in Australia as there were at the time of the Port Arthur massacre."

Victims were a 'much-loved family'

The ages of the children and adults have not been revealed.

State Member for Vasse Libby Mettam, whose seat incorporates Margaret River, said the deaths had already sent "significant shockwaves" through the town, and the community was hurting.

"This is a much-loved family, a big part of the Margaret River community," she said.

"Our thoughts and prayers go out to everyone that will be affected by this tragedy, including those first responders."

Former local councillor Felicity Haynes, who lives on a neighbouring property, said the family involved were "caring neighbours".

"They were just such lovely people," she said.

"They were a very socially-aware family, doing their best to create a safe community, and that is why it is so shocking to think that could be destroyed so quickly."

Dr Haynes said the family had moved to the property about three years earlier to start a farm, having earlier lived in Margaret River.

Another nearby resident, Freya Cheffers, said she was in shock.

Freya Cheffers lives near the Osmington home where seven people were shot dead. (Facebook: Freya Cheffers)

"Everybody's just devastated … I just prayed that I didn't know the family, as it turns out I kind of do."

Premier Mark McGowan described the incident as a "shocking tragedy".

"Clearly this is a distressing day for Western Australia," he said.

"It's a dark day for every family member or friend of the people involved."

Tragedy to have 'lasting impact'

Police said they were expecting to remain at the property for "an extended period of time".

A police mobile command unit has been set up at the property. (Supplied: The West Australian)

"We are trying to locate other members of the family and friends," Commissioner Dawson said.

"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."

Margaret River is a small town about 270 kilometres south of Perth, and the surrounding region — with a population of about 9,000 — is well-known for its beaches, wineries and craft breweries.

It is world-famous as a surfing destination, hosting an annual event of the World Surf League — the Margaret River Pro — and is a popular holiday spot among both West Australians and international tourists.

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Augusta-Margaret River shire president Pamela Townshend said the tragedy would send shockwaves throughout the small community.

"It really hits hard, because everybody has links with that family in one way or another, so it will have a ripple effect right through our community," she said.

"The good thing about a small community is that we all mobilise to support people who are in terrible distress, and there will be a huge community response … to support people affected directly and indirectly by this.

"My heart goes out to them and we will certainly provide as much support as we possibly can."

Anyone with information regarding the deaths, or who saw or heard anything suspicious in the Osmington area this morning, has been urged to call Crime Stoppers.