PERTH (AFP, REUTERS) - The grieving father of four children who were killed in a family mass murder and suicide case that rocked Australia said on Sunday (May 13) that their grandfather was to blame for what he called a planned shooting.

Mr Aaron Cockman's children - three boys and a girl aged eight to 13 - were among the seven people found dead by police on a rural property in Osmington, a tiny town of 135 people near the Margaret River wine region of Western Australia.

The bodies of their mother Katrina Miles, 35, and grandparents Peter Miles, 61 and his wife Cynda. 58, were also found at the property.

The four children were Taye, 13, Rylan, 12, Ayre, 10, and Kadyn Cockman, eight.

The tragedy is being treated as a murder-suicide and Western Australia Police Commissioner Chris Dawson told a press conference on Saturday that they are not looking for any suspects.

Mr Cockman, a local carpenter and builder, had been involved an acrimonious split with Katrina, which led her and the children to move in with her parents.

Mr Cockman told a press conference on Sunday that he had been full of anger after Mr Miles and his wife cut him off from seeing his children.

Mr Cockman said police had told him the children had died peacefully, with Kadyn in his mother's bed. "All the kids died peacefully in their beds," he said.

On Mr Miles, Mr Cockman said: "I think he's thought this through. I think he's been thinking this through for a long time."

Police have yet to confirm which family member was the shooter but said they were not searching for any suspects.

Three firearms licensed to Mr Miles were found by investigators.

Investigators have not revealed the motive behind the shootings, with Mr Cockman only saying he previously enjoyed a close relationship with Ms Miles' parents.

"He (Peter) was like my best friend and I still love who he was, but his mental attitude... there are some people you just don't get on the wrong side of...and that's Peter and Cynda," Mr Cockman added.

Mass shootings are rare in Australia, which has strict gun laws introduced after 35 people were killed in 1996 at Port Arthur in Tasmania.

The Osmington deaths are the worst mass shooting since then.

Western Australia state Premier Mark McGowan, who travelled to Margaret River on Sunday, said he did not think gun laws could be further tightened but he would await recommendations from a coronial inquiry into the shootings.