A Sydney teenager with special needs is set to face court again today following his arrest over his alleged preparation for a terrorist act.

The 17-year-old, who cannot be named for legal reasons, was charged about midnight on Tuesday, after police were alerted to several of his social media posts.

His matter was mentioned in the Parramatta Children's Court on Wednesday where it was adjourned until today. Bail was not applied for.

In court documents police allege the boy, from the city's south-west, created a map and deleted hard drives in planning for a terrorist act.

Police Commissioner Andrew Scipione told reporters the teen had also talked about killing an officer in social media posts.

The teen was arrested from his home in The Oaks, west of Camden, before being charged by the NSW Joint Counter Terrorism Team with preparation for, planning a terrorist act and using a telecommunications network with intention to commit a serious offence.

At a media conference in Sydney on Wednesday Mr Scipione implored officers to remain vigilant following the threat.

"Late last night, we put a warning out to our police again on the back of the most recent advice we've broadcast, that we know police and security services are currently targets," he said.

The teen’s father yesterday came out in his son’s defence.

"I understand the terrorist laws, he is no terrorist, he has got mental health issues, he was trying to start off a prank and that was it," he said.

He said his son had no relationship with radical Islam but he did have anxiety, depression and Asperger’s syndrome.

Mr Scipione said the teen had special needs and he expected "the courts will take all of that into account".

Mr Scipione said the teen had no known connection to any previous activity in terms of terrorism, but said there was still a threat.

"This was as a result of a post on a social media site, it talked predominantly about hurting a police officer, in fact, killing a police officer," he said.

"I understand that the individual involved has indicated he proposed to act in the very, very near vicinity in terms of time. He was allegedly going to conduct any act within a very short period of time."