But the witness, Rhys Dugdale, emphatically rejected any suggestion he was responsible for the vicious attack that resulted in his cellmate being placed in a coma.

Dugdale, who has been released from Canberra's Alexander Maconochie Centre since the December 2018 incident, testified in an ACT Magistrates Court hearing on Thursday.

He is the star prosecution witness in the case against Cedric Roberts, 23, and Brendon Walters, 27, who have pleaded not guilty to a charge of joint commission assault occasioning actual bodily harm.

Dugdale has claimed the pair repeatedly kicked and jumped on the victim after coming into their cell on the day in question.

The victim, who was in a coma for about 17 hours, has told the court he has no independent recollection of what happened to him.

Roberts' barrister, Jason Moffett, on Thursday suggested to Dugdale that he had "completely made up" the allegations against Roberts and Walters.

Mr Moffett referred to the evidence of the prison guard who discovered the injured victim unconscious, in his bed, under a doona.

The guard told investigating police Dugdale "wouldn't move" and was "freaking out" when he was told to leave the cell.

Mr Moffett asked Dugdale, who said he did not remember saying this, whether he had "put [the victim] into a state of unconsciousness by assaulting him" before trying to hide him with the doona.

"You were standing over [the victim's] body like a lion that had just killed its prey," Mr Moffett told Dugdale.

Dugdale rejected any suggestion he was to blame for the victim's injuries.

"That's bullshit," he said.

"[The victim] is a friend of mine. That's complete and utter garbage."

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Asked why he did not immediately tell the guard his friend had been attacked and needed help, he said other inmates, "including friends of the ones that just done [sic] it", were nearby at that time.

"I still had to live there," Dugdale told the court.

"I feared for my safety."

He added that he had not had any reason to attack the victim.

"I was living with the bloke," Dugdale said.

"He's f---en three times my size. How could I overpower him? It took two of them to do what they done [sic]."

Prosecutor James Melloy has previously told the court Walters had argued with the victim a short time before the bashing, giving the 27-year-old a motive to attack the man.

The victim has given evidence the "verbal" was about a tattoo gun, in which Roberts also apparently had an interest.

The attack was not captured on CCTV, with cameras at the Alexander Maconochie Centre not covering what happens inside cells.

Footage taken in common areas around the relevant time shows Roberts and Walters going into the cell shared by Dugdale and the victim.

But the video, played at the start of the hearing earlier this year, also shows what Walters' lawyer, Andrew Byrnes, has described as "a number of other suspects" coming and going.

The hearing, before Special Magistrate Jane Campbell, continues with each party's closing submissions on October 22.

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