Police forensics investigate the death of seven people in a suspected murder-suicide in Osmington, east of Margaret River, south west of Perth, Australia Friday, May 11, 2018. Seven people including four children were found dead with gunshot wounds Friday at a rural property in southwest Australia in what could be the country's worst mass shooting in 22 years, police said. (Richard Wainwright/AAP Image via AP)

Sydney - The grief-stricken father of four children, who were killed in a suspected murder-suicide by their own grandfather in a remote Western Australian farm, has spoken for the first time, saying "all the kids died peacefully in their beds."

"They looked all peaceful," Aaron Cockman told reporters on Sunday.

"I don't feel angry. I feel tremendous sadness for my kids."

Peter Miles, the 61-year-old grandfather, who was among the dead, is thought to be a prime suspect, although police have yet to formally confirm this.

Cockman's four children were among the seven members of the family found dead on Friday in Osmington in the Margaret River region, 270 kilometres south of Perth. The other two adults found dead were Cockman's estranged wife Katrina and her 58-year-old mother. 

Police have said they found three licensed guns on the property belonging to Miles and they are not looking for any other suspects. 

"Peter didn't snap. He's thought this through. I think he's been thinking this through for a long time," Cockman said. "How the hell Peter did that, I still can't figure out."

"I've lost everything in my life," he added.

Cockman said he had not spoken to his former father-in-law "since they cut me off from my kids." He also called Miles "an awesome man, before this all blew up."

"He was like my best friend."

"If it wasn't for Peter, I wouldn't have Katrina, I wouldn't have her kids... So it's not some random guy off the street who's taken them away from me - he gave them to me and now he's taken them away."