Undercover police officers stopped the boy for allegedly shoplifting at the station on Monday afternoon, but when they searched his backpack, they allegedly discovered the gun.

Officers from the Police Transport Command were performing covert duties about 3.30pm as part of an operation targeting property offences around the station.

The officers allegedly saw a group of teenagers enter a shop on the concourse, and the 15-year-old boy allegedly stole a bottle of soft drink.

The officers stopped the group and searched the boy and his bag, allegedly finding the unloaded pistol and a bottle of soft drink that had not been paid for.

The boy was arrested and taken to Sydney City Police Station, where he was charged with possessing an unauthorised pistol and stealing from a retail store.

He was refused police bail and is due to appear in Bidura Children's Court on Tuesday.

In a separate incident, a 25-year-old man has been charged after he was allegedly found carrying a mini Maglite torch that had been modified to fire ammunition.

Police said they stopped a black Toyota Aurion on Bonds Road at Riverwood about 2pm on Monday and searched the car. During the search, they found the torch that they allege had been modified to fire .22 calibre long rifle ammunition.

The driver was charged with possessing an unauthorised, prohibited firearm, and other firearm-related offences.

He was refused police bail and is due to appear in Burwood Local Court on Tuesday.