A teenage boy arrested over an alleged terrorism plot in Sydney was convicted of possessing a firearm just four months ago.

The 15-year-old boy, who cannot be named for legal reasons, is one of five males arrested on Thursday over an alleged plot uncovered in December 2014 to attack an Australian Federal Police building.

The boy was arrested at his family home in Georges Hall on Thursday morning and is expected to appear in Parramatta Children's Court.

In December last year, just days before police uncovered the alleged plot, he featured in a photo holding a calibre .30-30 rifle in Chester Hill between 8pm and 9pm.