When interviewed by the IPCA, a policeman identified only as Officer A said he gave chase when Gemmell fled up the driveway of a nearby residential property.
He could not remember if he took the keys out of the ignition first, but said it was his usual practice to do so.
"When Officer A reached the road, he saw [Gemmell] get into the police car through the driver's door and close it. By the time Officer A reached the driver's door, [Gemmell] had locked the doors and the engine was running. "
Officer A later said: "It was at this point I ran my hand over my vest and, with horror, realised that I had left the keys to the police vehicle inside it."
He yelled at Gemmell to get out of the car and started trying to break the window with his boot.
Officer B, who had also been involved in the chase, showed up and smashed the window with his torch.
Officer A tried to use his taser on Gemmell, but the probes did not work due to the puffer jacket Gemmell was wearing.
The IPCA found the use of the taser was reasonable.
Gemmell drove off in the police car and was followed by another pair of officers in a different patrol vehicle.
He rammed the other car before making a U-turn and leaving the scene. Once the other officers had armed themselves and picked up Officer A, they followed after Gemmell, coming across the stolen car abandoned, with lights and sirens going.
"When Officer A looked inside the car, he saw that the pistol lockbox was open, with the keys in the lock, and both pistols missing."
The police helicopter and Armed Offenders Squad were called out for the manhunt that followed, and Gemmell was arrested three days later.
One of the guns was found at the time of his arrest, while the other was only recovered three months later.
Police did not have any policy at the time for officers to remove keys from the police car ignitions, so Officer A did not commit any policy breach, the IPCA found.
But it said it was Officer A's responsibility to do it.
"His failure to do so was the only reason [Gemmell] was able to steal the car and the Glock pistols.
"The Authority, however, is mindful of the circumstances giving rise to this omission. [Gemmell] was clearly intent on evading Officers A and B, and his actions in ramming their patrol car and alighting from it were unexpected and fast-moving. Officer A's intention was to arrest and apprehend Mr X and it was reasonable that he reacted by giving chase immediately."
Since the incident, police have introduced a national instruction explicitly telling employees to ensure the ignition and firearm security keys are not left in the vehicle when unoccupied, and to lock and secure the vehicle.
Police are also developing a training package on vehicle security for front-line officers.
In a statement released today, police acknowledged the IPCA report.
"While there was no specific policy requiring officers to remove keys from their vehicles, police accepts that Officer A's actions enabled the offender to steal both the vehicle and the firearms," Southern District Commander Superintendent Paul Basham said.
"This was an unexpected and fast-moving situation, however changes have since been made, at both the district and national level, to help ensure this type of incident is never repeated."
Gemmell was convicted and sentenced to more than three years in prison.
