A gun seized by police after it was used in a 2002 murder was found in the possession of criminals 11 years later.

The murder weapon was seized by police a second time after a 2013 raid on a Sydney home, the Daily Telegraph reports.

The NSW Police Integrity Commission (PIC) are investigating how the weapon managed to end up back in the hands of civilians after being placed into police possession.

The discovery comes in the wake of the PIC uncovering the plot of a female police officer to help her former partner break into a police station in NSW to steal service weapons.

The officer was reportedly discovered after she illegally accessed the police database before sharing confidential information to the former partner.