The grieving father of four children who were killed in a family mass murder and suicide case that rocked Australia said on Sunday their grandfather was to blame for what he called a planned shooting.

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 the small town of Osmington near the Margaret River wine region of Western Australia.

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

Cockman, who told reporters in Margaret River he was estranged from Katrina, said "Peter didn't snap".

"I think he's thought this through. I think he's been thinking this through for a long time," Cockman added in his first public comments since the shootings.