The devastated father of the four children killed in the Margaret River family shooting says he believes he understands why their grandfather, presumed to be the one responsible, shot them as they lay in their beds.

Peter Miles, 61, his 58-year-old wife Cynda, their daughter Katrina, 35, and her four children with estranged partner Aaron Cockman – daughter Taye, 13, and sons Rylan, 12, Arye, 10, and Kadyn, eight – were found dead at the Miles’ rural Osmington property in Western Australia on Friday.

They had all been shot by firearms licensed to Mr Miles.

“I still love who Peter was,” Mr Cockman told reporters in nearby Margaret River on Sunday, speaking publicly for the first time since the tragedy.

View photos Aaron Cockman has said that he believes his children’s grand Source: AAP More

Mr Cockman had been embroiled in a bitter dispute with Katrina about access to the children, who were home-schooled, and blamed Peter and Cynda for that, saying they “cut me off from my kids”.

“If it wasn’t for Kat’s parents … I would still be with her. I miss her so much, let alone the kids.”

He said the apparent murder-suicide was “totally” Peter’s decision and speculated about the pressure that had mounted on him in the lead-up to the tragedy.

Mr Cockman said Peter had lost a son years ago and couldn’t bear the thought of losing the other, Neil, who is gravely ill with a kidney illness.

View photos Katrina Miles with her daughter Taye, 13, and sons Rylan, 12, Arye, 10, and Kadyn, eight. Source: AAP More

“There’s no way possible he can lose another son,” Mr Cockman said.

“He’s just gone ‘right… I can’t live anymore so this is it for me. But I need to take out everyone with me because that will fix the whole problem.’

“And he’s fixed the whole problem.”

Readers seeking support and information about suicide prevention can contact Lifeline on 13 11 14.

Suicide Call Back Service 1300 659 467.