The authority's report, released today, said police were first warned Hartley had possession of a firearm when a woman reported being assaulted and left at the side of the road at Levin on February 14, Valentine's Day last year.
The woman told a police central communications staff member that Hartley had threatened to shoot her and had access to firearms, although she had not seen them herself.
Police placed Hartley's home under observation and decided to enter it on February 24. The officers were armed but were not wearing ballistic body armour.
One occupant was arrested for obstructing police before the officers entered the house, where Hartley had overturned furniture to barricade himself in a bedroom.
He refused to come out despite repeated requests and a failed attempt to force the door open.
One of the officers could see Hartley through a hole in the door, standing in the middle of the room and apparently unarmed.
When Hartley started to move as though intent on harming himself, the officer forced the door open.
Hartley then partially hid in a wardrobe and took hold of a rifle, which he pointed at one officer before training it on another.
The first officer immediately stepped out of the room and drew his Glock pistol, while the other, who was two metres from the barrel trained on him, also drew his Glock and fired two rounds through the wardrobe door.
Hartley's rifle was found to be fully loaded with eight rounds, including one in the breach which was cocked and ready to fire at the pull of the trigger.
In his findings, IPCA chairman Judge Sir David Carruthers said police had used sound judgment and complied with the law in deciding to use potentially lethal force.
"The officer was justified in using force to defend himself and the force was not excessive in the circumstances."
He found there was no opportunity for the officers to consider other tactical options available to them.
There was no undue delay in the officers giving Hartley first aid and seeking medical help after he was shot.
But the report was also critical of the officers and their supervisor for failing to wear ballistic body armour while armed.
The IPCA recommended all staff be reminded of their obligation to wear armour while carrying firearms, in accordance with police policy.
The IPCA was also critical that police did not update the national intelligence application on February 14 to record information about Hartley's alleged access to firearms.
The authority said it was satisfied that matter had been dealt with appropriately by the police district.
