Emery moved over to the driver's seat, locked the doors and drove off.
He shouted for the Emery to stop and fired two shots at the police car's tyres with the Glock pistol he was carrying.
Emery escaped anyway and the police car was found abandoned that evening with the officer's M4 rifle and ammunition still secured in the boot's gun safe.
Emery was found three weeks later and charged.
The IPCA said the situation would not have occurred if the officer had appropriately assessed the situation and mitigated any potential risk posed by Emery.
"However, he failed to remove the car keys, enabling him to escape in the Police car. The Authority has determined that, at the time the shots were fired, the man [Emery] did not pose an immediate risk of serious harm or death.
"The officer, therefore, did not have justification for firing at the vehicle," said authority chairman Judge Colin Doherty.
Prior to the theft of the car the officer had been involved in a number of incidents which gave him concern about his safety. He decided he was going to routinely carry his pistol despite police policy which prohibits general arming of officers.
His supervisor supported his decision. During a conversation with the District Professional Conduct Manager the officer was told he should not generally arm himself and was reminded of the rules.
The authority found the officer should not have routinely carried his pistol and did not have justification for carrying it on the day of the incident and that the officer's supervisor did not have authority to allow him to be armed as a matter of routine.
It also found he was not justified in shooting the police car either in self-defence or to prevent the suspect's escape.
The Area Commander was also reprimanded for not addressing the officer's breach of the rules by carrying his pistol following the incident. Nor did he clearly instruct his supervisor to tell the officer to stop carrying it.
Police, responding to the finding, said officers had addressed the areas for improvement highlighted by IPCA.
An employment investigation was also launched as a result of the incident and the officer involved is now based in Rotorua.
"Murupara is like a lot of other rural communities where staff work by themselves a lot of the time," police said today.
"The work can be quite challenging at times and due to the remote locations in a critical incident immediate assistance is not always available.
"The decision making of the officer involved in this incident was driven by a desire to keep himself and the community safe.
"However, his decision to routinely carry a firearm did not follow police policy. All officers are able to carry a firearm after a risk assessment of the situation faced.
"We acknowledge the correct process was not followed by the officer involved or the supervising officer when deciding to routinely carry a firearm."
