Two people, including a 15-year-old boy, were arrested Thursday in Sydney over a terror plot targeting a government building, with authorities expressing alarm at the age of those being radicalized.

Two people, including a 15-year-old boy, were arrested Thursday in Sydney over a terror plot targeting a government building, with authorities expressing alarm at the age of those being radicalized.

The teenager and 20-year-old man were seized at their homes and charged with one count each of conspiracy to conduct an act in preparation for a terrorist act.

Three others currently in jail were also expected to be charged with the same offence, police said.

The arrests stem from evidence gathered during pre-dawn raids in Australia late last year in which 15 people were taken into custody and an alleged plan to kidnap and behead a member of the public was uncovered.

Australian Federal Police's Deputy Commissioner for National Security Michael Phelan said the plot linked to Thursday's arrests was not new, but related to last year's operation where material about targeting a government building was first discovered.

"Last December, there were a number of documents seized as related to the search warrants we undertook in Sydney and those documents clearly talked about a plan and there was government buildings named in those plans," he said, adding that one of them was the Australian Federal Police offices.

"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."

The arrests were part of Operation Appleby, a rolling investigation into persons suspected of being involved in domestic acts of terrorism, foreign incursions into Syria and Iraq and the funding of terrorist organizations.