A Sydney man accused of trying to source a gun and suicide vest in order to carry out a lone wolf "mass murder" terrorist attack has pleaded guilty.

Tamim Khaja pleaded guilty to one count of planning, or preparing, a terrorist attack in what was supposed to be the first day of his criminal trial in the Supreme Court.

The Macquarie Park man was just 18 years old when he was arrested by Counter Terrorism police in a Parramatta car park in May 2016.

Police allege he was trying to source a gun and a suicide vest and had been scouting potential sites to target in an "imminent" attack.

Court documents tendered on Monday reveal that between May 11 and 18, Khaja allegedly conducted reconnaissance of potential targets, including the Timor Army Barracks at Dundas in Sydney's north-west and the Sydney West Trial Courts at Parramatta.

The documents also allege Khaja tried to get hold of a flag of the Khalifa, understood to be the Islamic State group, and had sought guidance and assistance from a contact on how to carry out an attack.

Judge concerned about Khaja's deradicalisation

Tamim Khaja will be sentenced early next year after pleading guilty to terror offences. ( ABC News: Vincent de Gouw )

Khaja is currently being held in custody under one of the most high-risk classifications for a prisoner.

But his barrister, Greg Scragg, told the court his client had been a "model prisoner" and that he was about to begin a deradicalisation program in jail.

Justice Desmond Fagan expressed concern about Khaja's prospects for deradicalisation, saying he was planning what was "effectively a mass murder attack".

"The idea that a person can be rehabilitated from such views of religious zeal by a program needs some substantiation," he told the court.

"My real concern with offences of this nature is the risk to the community and the downside of that risk is extremely grave.

"When a person is convinced of a philosophy to the point of considering it warrants mass murder, one is really concerned."

Khaja's defence team will submit a psychiatric report, character evidence from family and friends and a report from Corrective Services NSW ahead of his sentencing hearing.

Khaja is expected to be sentenced in February 2018.

ABC/AAP