The police watchdog has found an officer should have removed the keys from his police car to stop it being stolen, despite being involved in a fast-moving situation.
In August, Hori Gemmell stole the police vehicle and two Glock pistols in Gore following a foot-chase.
He was caught after a three-day manhunt and later jailed for three years.
The Independent Police Conduct Authority (IPCA) says taking the keys out of the ignition would have stopped the theft of the car and two pistols.
"At the time of the incident, there was no specific policy requiring officers to remove ignition keys when leaving a police car unoccupied."
The incident unfolded after Gemmell drove past the two officers in a car missing its front grille and number plate, the IPCA says.
"Officers caught up with him when he stopped in a residential street. They parked behind him, and the man reversed ramming the front of the police car before getting out and running into a residential property.
"As he ran back onto the street, the man got into the driver's seat of the patrol car, which still had its keys in the ignition. The key to the car firearm safe was on the same key ring.
"Despite efforts by the officers to stop him, the man was able to drive away. He rammed a second Police car before abandoning the patrol car in a nearby street. He took with him two Glock pistols from the locked box in the patrol car."
One of the pistols was located when Gemmell was arrested three days later, while the other was found three months later.
"The officer was focused on apprehending the offender in a fast-moving situation," IPCA chair Judge Colin Doherty said.
"He inadvertently forgot to remove his keys from the patrol car, which unfortunately enabled the man to steal the car and the pistols locked in it."
Police say officers are now required to remove keys from an unoccupied vehicle and a training package on vehicle security is being developed for all front line staff.
"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," said Southern District Commander Supt Paul Basham.
"Southern District Police undertook a review of its practices and made improvements to the manual handling of car and gun safe keys.
"The IPCA also found the use of a taser during the incident was reasonable under the circumstances."
