Constable Zachary Rolfe, 30, has pleaded not guilty to murdering Kumanjayi Walker, 19, after the teen stabbed him in the shoulder with a pair of scissors on November 9, 2019.
Rolfe fired three shots into Mr Walker as he resisted arrest in a room at his grandmother's home in Yuendumu, 290km northwest of Alice Springs.
Expert witness Detective Senior Sergeant Andrew Barram reviewed Rolfe's body-worn camera video and says the constable should have ordered the teen to show his hands when he first came into view.
"They should not have gone (into the house) if they suspected it was Mr Walker," he told the Northern Territory Supreme Court in Darwin.
"He is clearly a high-risk offender with a propensity to previously arm himself."
The former officer-in-charge of the NT Police operational safety section also said Rolfe and his partner Sergeant Adam Eberl, then a constable, should not have allowed the teen to get so close.
"The first thing that is immediately apparent when Mr Walker comes into view is that he has got his hand in his pocket," Det Sen Sgt Barram said.
"They have put themselves in close proximity to someone that they suspect was armed."
Asked if Rolfe and Sgt Eberl had put themselves in danger by their actions, Det Sen Sgt Barram said: "Yes, they did".
"No distance between them. Limited time to be able to react. Completely goes against our training.
"It is all about not rushing in."
NT police were trained to "stop and step back for a moment. Assess and reassess. Do you really need to go in there?" he said.
"It is taking a cordon and containment approach to everything we go to."
Rolfe and three other officers were sent to the remote Indigenous community to assist local officers with general policing duties.
They were also ordered to arrest Mr Walker at 5.30am on November 10 when he was likely to be sleeping and easily taken into custody.
Instead, they found the teen about 15 minutes after leaving the local police station where the officer-in-charge Sergeant Julie Frost has said she handed the men a printed page outlining the arrest plan.
Rolfe walked into a dark room and shot Mr Walker about a minute later. The second fatal shot ripped through the teen's spleen, lung, liver and a kidney.
The Crown says Rolfe and his team were "intent" on finding Mr Walker after watching a video of him violently threatening two other policemen with an axe on November 6.
It has conceded the first shot, which was fired while Mr Walker was standing and wrestling with Sergeant Adam Eberl, was justified.
But it says the second and third shots, which are the subject of the murder charge, went "too far".
The trial continues on Tuesday.
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