In November 2016, Risto Petteri Rummukainen was driving a Toyota HiLux on Gundaroo Road about 20km south of Gunning when he swerved onto the wrong side during a bend and smashed into a Hyundai, killing the other driver.

Rummukainen, who is in his 40s and was a self-employed carpenter, had been working at a property in Gunning and had some alcohol beforehand, but experts could not establish whether he was over the legal limit or not.

During a police interview four days after the crash, Rummukainen said he had vague memories of seeing the victim slumped on the steering wheel and only recalled waking up at Canberra Hospital.

"I tried to think that time of day, you know, possibly could have been a bit of sun in the eyes," he said.

"Cause, cause I don't know exactly where the accident happened so I got no idea."

Another driver had seen the crash in his rear-view mirror.

Rummukainen stood trial in the NSW District Court for one count of dangerous driving causing death and in April 2019, a jury convicted him of the charge, which has a maximum sentence of 10 years' jail.

He was sentenced one month later to three years' jail with a non-parole period of 18 months.

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At trial, the Crown's case was that Rummukainen driving on the wrong side was sufficient to be dangerous as required by the charge.

In his closing address, defence counsel argued the critical question was why the car was on the wrong side.

The lawyer said there were a number of possibilities, such as driving on the wrong side because of "an innocent mistake", momentary lack of attention, or avoiding wildlife on the road.

He also argued for the first time during his closing about the possibility Rummukainen may have, without pre-warning or fault on his part, had "a microsleep, just for an instant".

"With such a thing as dangerous driving by falling asleep, it's not the act of falling asleep that's dangerous - that means that whatever you do afterwards is involuntary. It's the act of knowing that you're too tired to drive that's dangerous," he said.

The trial judge took issue with that proposition, saying there was no evidence Rummukainen had fallen asleep and he directed the jury to disregard that argument.

In October last year, Rummukainen and his lawyers filed for appeal on one ground - the trial judge erred in law in removing the possibility he was asleep.

Their defence was based on a High Court decision establishing that a driver who falls asleep while driving cannot be found guilty of dangerous driving during the period of sleep because the driving is not a conscious and voluntary act.

In a recent NSW Court of Criminal Appeal judgment, two of the three justices dismissed the appeal.

Acting Justice Carolyn Simpson said there was no direct evidence about Rummukainen falling asleep that could be deemed a defence of honest and reasonable mistake.

She also said there was no evidence from which an inference about that could be drawn and that "there was not, throughout the trial, the slightest hint that such a suggestion would be made".

Acting Justice Simpson said the defence of honest and reasonable mistake being made late was unfair on the Crown who "would have had no opportunity to address the jury on the defence".

Justice Peter Garling said the trial judge's direction "was inappropriate and has caused a miscarriage of justice".

"The conviction ought to be quashed and a retrial ordered," he said.

Justice Garling said the evidence about that possibility was a matter for the jury, not the trial judge.

Justice Natalie Adams said she agreed with Acting Justice Simpson for the same reasons.

As part of his sentence, Rummukainen was disqualified from holding or obtaining a driver's licence for two years.

In an earlier proceeding, he had his appeal against the sentence imposed dismissed.

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