The victim needed life-saving surgery following the vicious attack last September and spent four days in intensive care.

On Wednesday morning, former drone company operator Jamie Stewart Kinsela pleaded guilty in the ACT Supreme Court to recklessly inflicting grievous bodily harm on the man.

Kinsela, who is also known as Jamie Graham, further admitted to the theft of the victim's mobile phone.

Agreed facts tendered to the court say the victim, who had once lived briefly with Kinsela, broke into the 50-year-old's Spence home on August 29 last year and stole some drugs.

This was apparently captured by CCTV cameras installed at the front of the house, alerting Kinsela and one of his friends, George Dritsas, to what had occurred.

Kinsela and Dritsas, the latter of whom police have been unable to locate, sent the victim "several threatening Facebook and text messages" over the next few days.

The pair also sat outside the home of a Latham woman the victim was renting a room from, telling her the victim had stolen from Kinsela and they were going to deal with him "in their own way".

They eventually found him about 9.20am on September 4 at the Latham shops, where Dritsas is said to have chased the victim into the Latham Turkish Food Bar.

The agreed facts say Dritsas punched, kicked and stomped on the victim inside the shop before taking the man's shoes and leaving.

The victim crawled behind the counter as Kinsela approached the business, telling the owner the victim had stolen $5000 from his home while his teenage daughter slept.

"Well, just get him out of my shop," the owner told Kinsela.

"I don't want any of this in my shop."

Kinsela obliged, heading behind the counter and kicking the victim before dragging him outside by the shirt and the hair.

"The victim resisted and pleaded with [Kinsela] not to harm him," the facts state.

These pleas fell on deaf ears, with Kinsela instead punching the victim in the back of the head and stealing the man's smartphone after he dropped it.

Dritsas also slapped the man, stomped on his right ankle five times "in quick succession" and kicked him in the face.

The attackers then tried to shove the victim into the tray of Dritsas' ute, but they stopped when a woman who lived across the road screamed at them that her children were scared.

The victim took the chance to scurry away as Kinsela yelled at him to "get out of Canberra" before the offenders drove away.

According to court documents, one of the victim's lungs collapsed as paramedics subsequently rushed him to hospital.

Upon arrival, the man was also found to have two broken ribs, a fractured right ankle, bruises, lacerations and other injuries.

"The victim underwent a life-saving procedure," the facts say.

The victim later told police Kinsela and "George" had attacked him, but he did not provide a formal statement.

CCTV cameras captured the assault, however, and police arrested Kinsela during a raid at his home the following day.

Officers also found four cannabis plants growing in the backyard, along with marijuana in the garage and under a deck.

Kinsela, who has been behind bars ever since, was almost unrecognisable on Wednesday as he pleaded guilty to the grievous bodily harm and theft charges from behind a big, bushy beard.

Prosecutor Sofia Janackovic told Justice Chrissa Loukas-Karlsson the Crown would file a notice declining to proceed with some other charges.

Kinsela had previously pleaded not guilty to allegations of aggravated robbery and attempted unlawful confinement.

The 50-year-old will be sentenced at a later date, with two transferred charges of drug possession to also be taken into account.

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