Officers found two adults dead outside a building on the property in Osmington, south of Perth and 20 kilometres north-east of the town of Margaret River.
The four children and another adult were found dead inside. The bodies had bullet wounds, and police discovered two firearms at the scene.
West Australian Police Commissioner Chris Dawson said the incident could only be described as horrific.
â€śThis 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,â€ť he said.
Officers from the Homicide Squad and Forensic Unit arrived from Perth on Friday. Police said they would not comment on possible motives, but were not seeking to apprehend a suspect.
“We have no information to raise concern about wider public safety issues,” Mr Dawson said.
Peter and Cynda Miles bought the 30-acre hobby farm in late 2014.
Mrs Miles has been a stalwart in the sustainability community, volunteering with the local Transition Network and leading recycling initiatives including Cynda’s Soft Salvage.
Her husband, Peter, has worked as a farmer and teacher and was formerly the manager of the Margaret River High School farm.
Fairfax MediaÂ understands their four grandchildren, aged between eight and 13, were schooled at the sprawling homestead, among gardens full of fruit trees and flowering natives.
Members of the community said the couple were civic-minded and respected members of a “happy, wonderful” family.
“It’s sending shockwaves through the whole community â€“ weâ€™re all linked in one way or another, every family,” shire president Pamela Townshend said.
Ms Townshend described Cynda Miles as “a lovely, open-hearted, generous woman; embedded and active in the community, committed to the sustainability movement and very well known”.
Neighbour Rob Broun, who lived across from the crime scene, said he had heard no unusual or loud noises in the night and did not know his neighbours as he only visited on weekends.
“I’d seen who I assume was the father out in a paddock one day, on a tractor, but I didn’t have the opportunity to say hello,” the semi-retired farmer said.
Nikki LeFebvre, a reporter for the Margaret River Mail, said the town was “absolutely reeling”.
“Itâ€™s passed in a bit of a blur … a mixture of grief and loss and shock and trying to find out facts as well,” she told Radio 6PR.
“Gossip goes quick; you need to let people know whatâ€™s going on, but also know we are dealing with a lot of grief.
“Thereâ€™s been a lot of talk about who has been involved … family is still to be informed,” Ms LeFebvre said.
“Families need to have talks with their kids tonight and explain whatâ€™s going on.”
She said the Osmington community was made up of farmers and people who had moved into the area for the rural lifestyle.
Ms LeFebvre said the community was awash with offers of help and support, from counselling to shopping trips and school runs.
“Weâ€™re hardy people and we come together pretty well,” she said. “We’ve met with bushfires, shark attacks, so many things, and its heartening to see that when something like this happens it’s all ‘what can we do?’ (and) how can we help?”
Neighbour Felicity Haynes told Radio 2GB gunshots had been heard, but no one realised what had happened until police began to arrive.
“All the neighbours are in shock … everyone along Osmington Road. I mean, this is a very quiet peaceful community, mostly rural blocks,” she said.
“We all know each other, all support each other â€“ in fact, the whole Osmington community was very much united against this coal mine that was going to be erected right opposite the place where this tragedy occurred.
“Despite all different political persuasions, and differences in wealth and culture weâ€™re all united here along Osmington Road in being a supportive, caring community. So for something to happen like this is quite horrifying.”
On Twitter, WA Premier Mark McGowan described the deaths as shocking.
“My thoughts are with the family and friends of the victims, and also with the first responders and investigators as they piece together this tragic set of circumstances,” he said.
Police said they expected to spend a long time at the property.
The shooting is Australia’s worst since Martin Bryant killed 35 people in the Tasmanian town of Port Arthur in 1996.
In 2015, NSW farmer Geoff Hunt killed his wife and three children before wading into a small dam and shooting himself.
Anyone needing support can contact Lifeline on 13 11 14, or beyondblue on 1300 224 636. Anyone with information is asked to call Crime Stoppers on 1800 333 000.
with Simone Fox Koob, Patrick Begley and AAP
Emma Young is a Fairfax Media journalist based in Western Australia, breaking news with a focus on science and environment, health and social justice.
Hamish is a reporter writing for WAtoday in Perth.