Richard Jason Reay, 46, pleaded not guilty on Monday to murdering Geoffrey Fardell, 52, while the two were locked in a cell together at the Mid Coast Coast Correction Centre near Kempsey in June 2019.

The NSW Supreme Court was told the death came to light about 6.15am on June 11 when an inmate delivering tea and coffee approached the cell.

Reay told him "the old guy" had "got down" on the floor and died overnight.

When a prison officer went to the cell minutes later, he saw Reay sitting on his bed with the cell's TV on and Mr Fardell's immobile body face down on the floor, covered by a sheet.

"He's ... dead, chief. He's gone," the accused told the officer, the court was told.

Crown prosecutor John Stanhope said a video recording as officers searched the cell captured the voice of a man - allegedly Reay - saying "I woke up about 1am, he's just laying there on the floor".

But police later noticed ligature marks "more or less running the full circumference" of Mr Fardell's neck, Mr Stanhope said.

In the 15 hours before the June 11 morning coffee run, neither man exited and no other person entered the cell.

Justice Robert Allan Hulme, who is hearing the trial without a jury, heard the Crown did not accept Reay's guilty plea to manslaughter.

That plea was based on a claim that Reay genuinely believed his conduct was necessary to defend himself - conduct he now accepts was not reasonable in the circumstances.

Justice Hulme is yet to rule if the Crown can use evidence it says shows Reay's "tendency to be violent ... without provocation" in the murder trial.

Reay was jailed in 2003 after striking a man with a baseball bat in the head for no apparent reason - an attack he told a witness would have been fatal if not for her intervention.

He was also held in segregation for three years in a Queensland prison and has a history of violence towards a dental nurse, police in a courtroom, prison officers and other prisoners.

Public defender Angus Webb said he sought to rely on evidence pointing to Mr Fardell's own tendency for violence.

The trial continues.

Australian Associated Press