On May 11, 2018, Aaron Cockman lost his all four of children after their grandfather shot them in their sleep before killing himself.

In an emotional interview on Channel 7's Sunday Night, the grieving father opened up about Australia's worst mass shooting since the Port Arthur massacre and explains the circumstances he believe led his former father-in-law, Peter Miles, to kill his family.

Taye, 13, Rylan, 12, Ayre, 10 and Kayden, 8, their mother Katrina Miles, and grandmother Cynda Miles were all found murdered in their beds at home in Osmington, a rural region near Margaret River in Western Australia

Cockman described the heartbreaking moment he found out his children were dead.

"I'm working on a job site, and I just looked at my phone then I hear this message from the police saying, 'Can we talk to you?'"

His colleagues learned of the news on the radio and asked if he had heard that four kids and three adults had died.

"My heart just dropped," he said. "And I said, 'Yeah, I think that's my kids'. I said, 'I've got the police coming here now.'"

"I still can't believe that this is even happening," Cockman said.

The once perfect family began to crumble under the pressure of a custody dispute, the suicide of Katrina's brother Sean, and life-threatening diagnosis of her other brother Neil.

But Cockman said the trouble really began when his wife moved in with her parents.

"The children started to be groomed to hate us and to hate Aaron," said Cockman's mother Kim. "And that … That was wicked."

Cockman said the dispute over care of the children came at a cost — causing strain on both families.

Meanwhile neighbour Felicity Haynes said Peter Miles was being treated for depression: "It was known that he was seeking psychological help and psychologists had prescribed antidepressants."

She said Cynda Miles was showing the strain when she returned for visiting her son Neil in hospital and had said there were "other things happening at home".

"I do think she was referring to the fact that Peter was being fairly irrational," Haynes said.

Cockman thinks his former father-in-law killed his family to "solve the problem" surrounding the shared care of the children.

Cockman's mother believes Miles killed the kids so they couldn't be with Aaron.

In a brief, but bizarre suicide note left by Miles read: "Ex-husband, Aaron Cockman to have house content. Peter Miles."

If anything positive is to come from this devastating situation, Cockman hopes it's that family courts will make "quicker decisions" and easier ways for court orders to be enforced, as he is convinced it was the long-drawn out court battle that led to Mr Miles' demise.

"I'm going to put all my effort into that," Cockman said. "I've got nothing else now."

WHERE TO GET HELP:

If you are worried about your or someone else's mental health, the best place to get help is your GP or local mental health provider. However, if you or someone else is in danger or endangering others, call police immediately on 111.

OR IF YOU NEED TO TALK TO SOMEONE ELSE:

• LIFELINE: 0800 543 354 or 09 5222 999 within Auckland (available 24/7)
• SUICIDE CRISIS HELPLINE: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• YOUTHLINE: 0800 376 633 ,free text 234 or email [email protected] or online chat.
• NEED TO TALK? Free call or text 1737 (available 24/7)
• KIDSLINE: 0800 543 754 (available 24/7)
• WHATSUP: 0800 942 8787 (1pm to 11pm)
• DEPRESSION HELPLINE: 0800 111 757
• SAMARITANS – 0800 726 666.