Authority chairman Judge Sir David Carruthers said the officers were faced with a "very real risk of death or serious bodily harm" during the incident as Marshall had pointed a firearm at them and squeezed the trigger - but it failed to fire.
"As soon as officers entered the warehouse, Mr Marshall confronted them. He presented a very real risk of death or serious bodily harm to police and needed to be urgently stopped.
"The officers who shot at Mr Marshall were justified in doing so," Judge Carruthers said in a statement.
On arrival, police initially used a loudhailer to get Marshall's attention before smashing their way through the front door.
After police got inside, Marshall walked out of an office and after being told to get down on the ground he ran to the back of the building.
Marshall then picked up a pump action shotgun, pointed it at police and pulled the trigger but it failed to fire. Marshall then pointed the shotgun towards the ceiling and racked the chamber, causing a bullet to fall to the floor.
An officer yelled at Marshall to drop his gun. Marshall ignored him and instead tried to load another round.
Believing Marshall was going to shoot him, the officer fired five shots.
At the same time another officer, who saw Marshall holding the shotgun and believed that the officer would be shot, fired three shots at Marshall.
Officers immediately performed CPR but he died at the scene.
A number of firearms, either operational or in parts, and machinery used for the manufacture of firearms were located during the subsequent search of the warehouse.
Marshall's family were yesterday visited by the IPCA about their findings.
Nelson Marshall told the Herald today "there's for and against" about the decision.
"I don't think it was handled as well as it could have been but the fact was that Nick did pick up a shotgun, so what do you do?"
Marshall Snr said he had seen the shotgun that his son was holding in the shop previously but doubts he would have been wanting to shoot police on purpose.
His son had been the victim of a home invasion by Mongrel Mob gang members a month earlier and Marshall suspects his son would have thought it was them returning - and not AOS members.
"He'd thought they'd come back again and so he went out there to grab the gun, knowing it was unloaded he grabbed some shells and was trying to load it when they shot him and they thought he was trying to fire it.
"And he was loading it while he was trying to figure out what was going on and who was attacking him and basically showing them that he had a gun, so don't come any closer, and then the cops thought 'oh a gun' so they shot him, bang, bang, bang, bang. Five shots. Three from another, but they all missed."
He doubted Nick would have heard police with their loudhailer out the front of the premises, as there was another business in front of them, they stormed inside straight away and he would have been listening to music.
The saga isn't over the family though and Marshall said they would now await the coroner's hearing.
The family would now advocate for all Armed Offenders Squad members to be equipped with cameras on either their guns or their helmets so it recorded exactly what happened - and the public didn't have to just rely on their word instead.
"With no cameras, it's only their word against everybody else's."
The warehouse was also tested for P but the results came back below the normal range. A post-mortem examination revealed Marshall had a low level of P in his system, the equivalent of having had one smoke, he said.
Waikato police district commander Superintendent Bruce Bird said his staff had little choice but to resort to the use of firearms after Marshall refused requests to get on the ground and instead pointed a pump-action shotgun at them and attempted to chamber rounds.
"As the authority noted, staff were presented with a very real risk of death or serious harm by Mr Marshall who needed to be urgently stopped," Bird said.
"Due to the rapidly evolving situation, other tactical options were neither viable nor appropriate, with Mr Marshall's actions ultimately leaving staff with no other option but to use lethal force to keep themselves and others safe, with the authority finding that the shooting was justified."
"While this was a tragic end that nobody would have wanted, I commend the officers who responded and who acted with professionalism and considerable courage," Bird said.
"This was a highly-charged and dangerous situation and I am grateful that there were no further injuries or yet more tragedy."
