Johnson had pleaded not guilty to murder, but guilty to manslaughter at the outset of the judge-alone trial in Newcastle Supreme Court, admitting to slashing Mr O'Brien in the throat but claiming he was acting as a result of excessive self-defence.

Mr O'Brien, 50, his family and friends spent Boxing Day 2019 at his home in Silsoe Street, Mayfield, and were in the backyard when Johnson joined them later in the evening.

The group were all drinking alcohol and socialising happily for a number of hours, witnesses have said.

But sometime after Mr O'Brien's parents had gone to bed - leaving only Mr O'Brien, Johnson and Billy Gee in the backyard - something happened that prompted Johnson to grab a weapon and slash Mr O'Brien across the throat.

Johnson, who is represented by barrister Chris Watson and solicitor Mark Ramsland, did not give evidence and so the trial focused almost solely on Mr Gee to determine what happened in those crucial moments before the stabbing.

The Crown case was that Johnson was "harbouring resentment and intent on seeking retribution" when he brought a knife to the BBQ and accused Mr O'Brien of having an affair with his partner.

"The accused has come to the house at the very least as a provocateur in a situation where he is going to ferment conflict and confrontation, "Crown prosecutor Neil Adams, SC, said during his closing address.

Those claims were based on the evidence of Mr Gee, who had said Johnson accused Mr O'Brien of "sleeping with his partner" and then reacted by lashing out with a backhanded slash of Mr O'Brien's throat when he was told Mr O'Brien had "higher standards than that".

But during his judgment on Thursday, Justice Richard Cavanagh said he did not accept much of Mr Gee's evidence, outlining the varying inconsistent versions he had given about crucial aspects of the case.

He said Mr Gee's recollection of what was said before Mr O'Brien was stabbed changed "depending on who he was speaking to" and pointed out Mr Gee had waited 11 months to tell police Mr O'Brien had stood over Johnson and grabbed him by the shirt before the stabbing.

And he said the fact Johnson had spent hours at the house happily socialising was inconsistent with him coming to the house as a "provocateur".

Justice Cavanagh rejected the prosecution case and said it was more likely Johnson had "lashed out recklessly" during some sort of physical altercation with Mr O'Brien and possibly Mr Gee.

He found Johnson not guilty of murder, but guilty of manslaughter, determining Johnson believed it was necessary to defend himself but it was not a reasonable response in the circumstances.

Johnson, who appeared in court via AVL from a remote room at the courthouse due to COVID concerns, will be sentenced at a later date.

Through Mr Ramsland, Johnson wished to apologise to Mr O'Brien's family, saying "I never thought that would happen".