A 15-year-old boy was one of two people arrested over a terror plot during dawn raids in Sydney.

The teenager and a 20-year-old man are believed to have been planning attacks on government buildings, including Australian Federal Police.

Investigators say they were also targeting random civilians.

The pair, who were arrested at their homes, have been charged with conspiracy to conduct an act of terrorism.

Three others currently in jail are expected to be charged with the same offence. The arrests were connected to raids late last year in which 15 people were detained.

Those raids shed light on plans to hit a government building and behead a member of the public.

National Security chief Michael Phelan said: "Last December, there were a number of documents seized...and those documents clearly talked about a plan and there were government buildings named in those plans".

"As a result of putting all of that information together, working through those documents, putting physical and electronic surveillance together, we were able to build a case of conspiracy for five people involved in the preparation of these documents".

Asked how far along the plot had been, Mr Phelan replied: "Enough for us to disrupt it at the time in December".

The arrest of a teenager comes after a radicalised 15-year-old named Farhad Jabar was blamed for the killing of police employee Curtis Cheng.

Deputy Police Commissioner Catherine Burn said: "It's disturbing that we're continuing to see teenage children in this environment".

"There's absolutely no doubt that the fact we're charging a 15-year-old with a very, very serious offence, one that has a maximum of life imprisonment, this is concerning not only to us in law enforcement but should be concerning to everybody".

"How they've become radicalised we don't actually know. We don't know how the 15-year-old has got to the point where we will allege he got".

Police said the latest suspects arrested are "associates" of those accused of being behind Mr Cheng's murder.