Constable Zachary Rolfe, 30, has pleaded not guilty to murdering Mr Walker, 19, during a failed arrest attempt in Yuendumu, 290km northwest of Alice Springs.
Rolfe fired three shots into Mr Walker's back and torso on November 9, 2019 after he had stabbed him with a pair of scissors while resisting arrest.
The former soldier, who served in Afghanistan, says he did not initially regard the teen as a threat when he spotted him inside his grandmother's home.
That changed in a second after Rolfe and Sergeant Adam Eberl identified Mr Walker and attempted to handcuff him.
"Kumanjayi started resisting. He raised his arms and started striking me around my head and neck area," he told a packed Northern Territory Supreme Court on Thursday.
"Kumanjayi struck me twice on the top of the head in a hammer fist motion.
"At that point I looked at his hands. That was the first time I identified that he had a metal blade. I saw him holding a blade in a dagger-like grip. I immediately feared for my life."
Rolfe said Mr Walker then stabbed him in his left shoulder. He said he instinctively "jabbed" the teen's face and reached for his firearm.
"His left hand was already on my Glock. I twisted my hips back as we are trained to do to knock that hand off my Glock and stepped back," he said.
"Kumanjayi's focus turned to Eberl and I immediately feared for Eberl's life. Kumanjayi started stabbing Eberl in the chest and neck area.
"I drew my Glock and when it was safe to fire, still fearing for Eberl's life, I fired one round into the centre of mass of Kumanjayi."
Rolfe said the shot did not "incapacitate" the teen, and he and Eberl fell to the ground and continued fighting.
Earlier, the constable has told the court he first became aware of Mr Walker on November 7 in Alice Springs when the teenager was listed as an "active arrest target" for breaching parole and assaulting police.
"I characterised him as a high-risk offender, extremely violent, who was willing to use potentially lethal weapons against police," he said.
Rolfe said he viewed body-worn camera footage of the so-called "axe incident" which showed two officers freezing in "an extremely, potentially deadly situation".
Two days later he and three officers from the specialist Immediate Response Team were ordered to visit Yuendumu.
Rolfe and his team found Mr Walker 15 minutes after they left the Yuendumu police station. Rolfe fired his first shot about a minute later.
Mr Walker died about an hour after Rolfe's second shot ripped through his spleen, lung, liver and a kidney.
The Crown has conceded the first shot, which was fired while Mr Walker was standing and wrestling with Sgt Eberl, was justified.
But it says the second and third shots, which are the subject of the murder charge, went "too far".
Evidence presented to court has included that Mr Walker was pinned to the ground by Sgt Eberl and was a "low threat" when Rolfe pulled the trigger the second and third times. The Crown says the shots were unjustified.
Expert witnesses have said the duo should have never gone into the house and Rolfe had not acted as trained.
The trial continues.
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