The father of Jake Bilardi, the 18-year-old Australian schoolboy believed to have carried out a suicide bombing for Isis in Iraq, has spoken of feeling "totally responsible" for failing to help his son.

Bilardi, from Melbourne, is believed to have travelled to Syria to join the jihadist group, when he was unable to launch a terror attack on home shores.

His father, John Bilardi, said his son had been "a prize, a trophy" to Isis, who "used him for their own cause".

"I would just like everyone to know that the buck stops here with me. He was my son," he told Australia's Nine Network's 60 Minutes programme.

"I knew there was something not right with his behaviour. He had psychological or mental issues that should have been addressed and I feel totally responsible for that.

"I should have been there for him and he obviously needed help and, as a parent, I wasn't able to do that."

He described his son's difficult childhood, and said that he was a solitary child, prone to violent outbursts.

He described one incident.

"My wife at that time was calling out to me, screaming," he said in an interview aired on Sunday night. "I ran into the room where she was and she was backed up into a corner with Jake thrusting a pair of scissors at her."

He described having to physically restrain his son.

After a messy divorce, he said he lost contact with Jake, but they reconnected when Jake's mother died.

He said that Jake declared that he was becoming a Muslim, and would spend hours alone in his bedroom, where he would watch Isis propaganda videos.

"Just out of the blue he said, 'I've gone Muslim' and I... fell back in my seat," Bilardi said. "And being atheist myself and his mother was pure atheist, that shocked me to hear that."

He described his shock at seeing photographs of his son clutching a gun on pro-Isis social media accounts.

"Just to see him sitting there with that gun is just unbelievable," he said. "I just couldn't believe that was my son."

He said he held on to a memory of his son as a softly spoke, shy and intelligent boy, with a passion for video games and The Simpsons.

"That's the memory. The rest of it, as far as I'm concerned, can go to hell," he said.