"Shocking" CCTV footage has been released of an off-duty police officer being ambushed and "coward" punched in an Adelaide city laneway.

Key points: Wol Magot Door punched a police officer in the back of the head in November 2015

Wol Magot Door punched a police officer in the back of the head in November 2015 Door was handed a non-parole period of 18 months

Door was handed a non-parole period of 18 months The judge described the attack as "well executed"

In sentencing one of the men, District Court Judge Wayne Chivell said the community was "revolted by street violence" and "coward punching".

"The video recording is deeply shocking," he said.

"It is true that such behaviour has caused very serious harm and even death on occasions in the past."

Judge Chivell sentenced Wol Magot Door to two years and 10 months' jail with a non-parole period of 18 months after he admitted to aggravated counts of assault causing harm and theft.

Door pleaded guilty on the second day of his trial.

Judge Chivell said Door and two others were jumping up and hitting overhead retail signs along Rundle Mall early on November 1, 2015, causing one to drop to the ground and break.

He said an off-duty police officer was walking behind the group and started taking photos.

"That was both his right and his duty as a police officer. In one sense, police officers are never off duty," he said.

"Their oath is to keep the peace and that is not limited to occasions when they are on duty."

He said once the group got to the corner of Rundle Mall and Pulteney Street, they confronted the off-duty officer about taking photographs.

"He told you then that he was a police officer," Judge Chivell said.

Coward punch 'in every sense'

Judge Chivell said the group crossed Pulteney Street and turned down York Street as the off-duty officer continued to follow them as he spoke to police on the phone.

"He followed you into York Street and walked into an ambush," he said.

"There is no evidence that your movements as a group were pre-planned, but they were well executed."

He said Door hid in a small alcove on York Street and as the officer walked past him, following his two friends, he came up from behind him as the other two turned around.

"While the victim was concentrating on the other two men, you hit him with a forceful blow to the back of his head," Judge Chivell said.

The footage of the attack was released to the ABC. ( Supplied: Courts SA )

"He remembers nothing after that.

"But the video shows he stumbled and fell to the pavement. This was what has become known as a 'coward punch', in every sense of those words.

"While he was lying semiconscious and helpless on the pavement, all three of you then proceeded to kick and stomp on his head.

"At one point, you picked up his iPhone and used the edge of it as a weapon to hit him in the head."

Door stole the phone and the three of them fled.

Door had witnessed 'many atrocities'

The unconscious officer, who "still suffers psychological symptoms", was helped by members of the public and Door was arrested later that day.

The court was told that Door was born in Sudan and witnessed "many atrocities" as a child.

Judge Chivell said he lived in a Kenyan refugee camp with two siblings until he was 15 before migrating to Australia in 2008.

"I have read the letter from Reverend Kerr, who has provided me with more detailed information about the awful experiences you had in Sudan and later in Kenya," he said.

"The letter excites great sympathy for you."

He said Door was an "intelligent person", having completed Year 12 and partly completing a commerce degree at the University of Adelaide, as well as a nursing diploma.

A forensic psychologist told the court that Door had symptoms consistent with Post Traumatic Stress Disorder (PTSD).

"You have no previous conviction for violence — that is a significant factor," Judge Chivell said.

"You have overcome substantial obstacles to get where you have.

"It is a tragedy that you have now put yourself in the position you find yourself in."

Police originally charged another man along with Door over the incident, but he was acquitted in December last year.