Tamim Khaja was 18 when he was arrested in May 2016 during a joint New South Wales and federal police operation after he began to scope out potential target buildings around Sydney.
The Macquarie Park resident was expected to face trial in Parramatta Supreme Court from Monday but instead entered a last-minute plea of guilty.
Khaja, now 20, has admitted to attempting to obtain weapons and a flag of the Khilafah, and seeking guidance about how to pull off his lone-wolf plot, court documents show.
He obtained a $25,000 loan from the Commonwealth Bank and bought a solar panel battery charger for his phone in preparation.
Khaja also purchased a return airfare from Australia to Thailand and attempted to leave in early 2016 but was stopped at Sydney International Airport.
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It's thought his potential targets included the Parramatta District Court precinct and Timor Army Barracks in the city's northwest. His plan reportedly involved a suicide vest.
Defence counsel Greg Scragg requested a sentence date in February, saying Khaja would have been in a de-radicalisation program for some time by then and the court could consider evidence of its effectiveness.
"It's very much in the interests, in terms of the appropriate sentence [...] and the community, that this offender enter into such programs that are available to him," Mr Scragg told the court on Monday.
Justice Desmond Fagan said the matter should proceed promptly.
"If any weight is to be given to this I'd need to have evidence from a corrective services officer who would explain it to me," he told the court.
Khaja's plot was "effectively a mass-murder attack" in the interests of pursuing Islamic dominance of Australia, the judge said.
"The idea that a person can be rehabilitated from such views of religious zeal by a program needs some substantiation," Justice Fagan said.
He described as "extremely grave" the risk to the community and said Khaja had not acted in "a moment of rage or some passion" but in relation to a philosophy that he believed warranted mass murder.
Mr Scragg agreed the offence was extremely serious but said it was in the interest of justice that the young man have more time to prepare his case.
"Since he's been in custody, the overall assessment of him is that he's been a model prisoner," Mr Scragg said.
The defence team will submit a psychiatric report, character evidence from friends and family and material from corrective services when Khaja's matter returns to court on February 16.
Justice Fagan said he expected to pass sentence within two weeks from that date.
