The heartbroken father of an Australian schoolboy believed to have died while carrying out a suicide bombing for Islamic State said on Monday he felt "totally responsible" for failing to see that his son needed help.

Jake Bilardi, 18, from Melbourne, is believed to have carried out the attack in Iraq earlier this month, after allegedly ditching plans for attacks on home soil.

Speaking to the media for the first time since his son's death, John Bilardi said Jake had been "a prize, a trophy" to the jihadists 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 Nine Network's 60 Minutes program late on Sunday.

"I knew there was something not right with his behavior, 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."

Jake, a talented student, had been brought up an atheist but converted to Islam, reportedly shortly after the death of his mother.

"Just out of the blue he said: 'I've gone Muslim'," his father told 60 Minutes.

A blog widely attributed to Jake Bilardi explains the teen's switch from suburban schoolboy in comfortable Melbourne to violent jihadist ready to sacrifice his life for the cause.

The writer said he developed a "complete hatred and opposition to the entire system Australia and the majority of the world was based upon".

But John Bilardi, who had recently reconnected with Jake after losing contact during a messy divorce, said he struggled to reconcile the son he knew with the images of the pale and skinny teen in Islamic State propaganda.

"He was recruited online, and I knew nothing of this. I knew nothing of his radicalization," Bilardi said.

"To see him sitting there with that gun ... I just couldn't believe that was my son.

"It's hard to process this. But that wasn't him. He was just a shy, lonely young man. How he got to this I cannot even start to try and explain."

The family was struggling to come to terms with the situation, made harder by the fact they had no body to mourn over, he added.

The Australian government has increasingly been sounding the alarm about radicalized citizens, with about 90 thought to be fighting with the Islamic State group in Iraq and Syria.