The 19-year-old died after Rolfe, 30, shot him three times during a failed arrest in Yuendumu, 290km northwest of Alice Springs on November 9, 2019.
Rolfe is fighting a murder charge over the second and third shots, which prosecutors say went "too far" because Mr Walker was "effectively restrained" by another officer.
Senior Constable Anthony Hawkings was part of the immediate response team that set out to arrest Mr Walker about 7.06pm.
He ran from the side of the teen's grandmother's house to an open door after Rolfe fired his first shot 16 minutes later.
"I saw the two final shots out of the three fired," he told the Northern Territory Supreme Court on Thursday.
"I remembering seeing (Rolfe) holding a firearm in his right hand."
"There was a struggle going on between three people and there were people outside the house that I could hear and see, yelling."
Snr Const Hawkings said Mr Walker was laying on a mattress on the ground in a "semi-prone" position and Rolfe fired at his body from about 30cm away.
"The proximity would have been extremely close, within a foot," he said.
"Very, very close.
"I can not recall if he was on his stomach or if he was more so on his side, struggling."
Snr Const Hawkings' body-worn camera recorded him running towards the scene amid screams from Mr Walker's family and friends.
As he approached the doorway Rolfe can be seen silhouetted against a red wall stooped over Constable Adam Eberl and Mr Walker.
Rolfe fired his second and third shots after Mr Walker stabbed him in the shoulder with a pair of scissors.
Prosecutors have conceded the first shot, which was fired while Mr Walker was standing and resisting arrest, was justified.
But it says the fatal second and third shots when Mr Walker was laying on the ground went "too far" because he was "effectively restrained".
Mr Walker died from injuries sustained from one of those shots.
The trial continues on Friday.
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