The blog widely attributed to Jake Bilardi also reveals a "Plan B" to wage bombings on home soil in case he was prevented by Australian authorities from leaving to fight overseas.
Officials have not confirmed Bilardi's death but a photo purportedly from a propaganda video by the Islamic State group appears to show him sitting in the driver's seat of a car used in a suicide bombing this week in Ramadi.
Members of two mosques in Melbourne and friends of the teenager have confirmed his identity based on photographs circulating of him online, saying he converted to Islam after his mother died of cancer.
The 4,400-word online manifesto is authored by "Abdul Abdullah Al-Australi", the name widely reported as the jihadist nom de guerre taken by Bilardi after he left Australia last year.
"With my martyrdom operation drawing closer; I want to tell you my story, how I came from being an atheist school student in affluent Melbourne to a soldier of the Khilafah (Caliphate) preparing to sacrifice my life for Islam in Ramadi, Iraq," the blog reads.
"My life in Melbourne's working-class suburbs was, despite having its ups and downs just like everyone else, very comfortable," it says.
The blogger writes of sitting on the couch eating breakfast as a school child and watching a television story on the Afghanistan's Taliban.
"It was Channel 7's programme 'Sunrise' that I turned on most mornings, watching discussions such as, 'Another attack in America, should we be suspicious about the Muslims in Australia?' it says.
"I saw the Taliban as simply a group of proud men seeking to protect their land and their people from an invading force, while I did not necessarily agree with their ideology, their actions were in my opinion completely justified."
'VIOLENT GLOBAL REVOLUTION'
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He said it was his oldest brother, who was deeply interested in foreign affairs, who first mentioned the words "Osama bin Laden" and "Al-Qaeda" to him.
"But as I know he is unhappy with me being here, I can confirm for his sake that, no, he did not 'radicalise' me."
The writer speaks of his obsession with politics and how he undertook his own Internet research, boosted by a state government decision to give students laptops in classroom, allowing him to continue this work at school.
His research led him to feel disgust for the "lies and deception" of the West -- particularly for the conflicts in Iraq and Afghanistan.
"I transitioned from being a reluctant supporter of Islamic militant groups in different lands to become certain that violent global revolution was the answer to the world's ills," it says.
He developed a "complete hatred and opposition to the entire system Australia and the majority of the world was based upon".
The document speaks of the "Plan B" after a realisation that "increasingly-intrusive authorities in Australia" could impose a travel ban on him.
"This plan involved launching a string of bombings across Melbourne, targeting foreign consulates and political/military targets as well as grenade and knife attacks on shopping centres and cafes and culminating with myself detonating a belt of explosives amongst the kuffar (infidels)."
Once he realised that purchasing bomb-making chemicals would attract unwanted attention, he cancelled these plans and waited until he could leave the country undetected.
Greg Barton, a social science professor at Monash University, said while the level of English expression in the blog was beyond what should be expected from someone so young, he believed it was by Bilardi.
"It lines up. I think we are dealing with a precociously bright and idealistic kid who was a loner and who largely moved along this path by himself," Barton told AFP.
About 90 Australians are said by the Canberra government to be fighting with the Islamic State group in Iraq and Syria.
The government has increasingly been sounding the alarm about radicalised Australians. Last Friday two teenage brothers were stopped at Sydney Airport from leaving for an undisclosed Middle Eastern country.