Jordan Miller, now 24, was in 2022 found guilty of murdering his 18-year-old girlfriend Emerald Wardle at Metford in June 2020 after a jury found the psychosis he was suffering was caused solely by using LSD and cannabis.

He was later jailed for a maximum of 20 years, with a non-parole period of 13 years and is currently not eligible for parole until 2033.

But Miller's chances of having his murder conviction quashed and a special verdict of act proven but not criminally responsible entered have received a boost with a prosecution psychiatric expert agreeing with a new report from defence expert, psychiatrist Dr Olav Nielssen.

The development in the state of the psychiatric evidence was revealed in the NSW Court of Criminal Appeal late last week and comes nearly four years after Miller suffered the psychotic episode and killed Ms Wardle.

"The Crown expert agrees to the new report of Dr Nielssen," a DPP solicitor said. "That does not provide a complete answer to the appeal. There are other questions that need to be answered."

The expert reports are currently with higher-ups at the DPP and a decision is imminent about whether or not the full-day appeal in the state's highest court on April 5 will go ahead and whether the experts will be required to give evidence, as was planned when the details of Miller's all ground appeal and bid to call fresh evidence were revealed last year.

If the DPP concedes the grounds of appeal then the CCA could enter a special verdict or potentially order a re-trial to determine if Miller is not guilty of murder but guilty of manslaughter on the basis of substantial impairment.

As the Newcastle Herald has reported previously, there was no history of domestic violence between the couple and Miller - a university student working part-time - had no criminal record.

Instead, he was in a psychotic state and had lost touch with reality causing him to believe Ms Wardle - the woman he was in a loving relationship with - was a "demon" who was "sucking the life out of him."

There was no dispute during the trial that it was Miller who had killed Ms Wardle.

He confessed; first to police and then during his first appearance in court, repeatedly saying: "I am a murderer."

And the medical experts called to give evidence agreed he was in a psychotic state at the time of the killing.

The only issue for the jury to determine was what caused him to be in that psychotic state and made him believe he had no choice but to kill the young woman he loved.

Miller had pleaded not guilty to murder and raised a defence of mental health impairment, with Dr Nielssen giving evidence that he was suffering a first episode of psychosis in the form of an underlying chronic schizophrenia, which not caused solely by drug use.

Meanwhile, the prosecution said the psychosis Miller was suffering at the time of the killing was caused solely by using LSD and cannabis.

During his closing address, Crown prosecutor Lee Carr, SC, pointed to the reports of Professor David Greenberg who opined Miller's psychotic episode was "temporary", related to his use of drugs and it was too early to definitively diagnose him with schizophrenia.

And after deliberating for about 12 hours the jury rejected the evidence of Dr Nielssen and agreed with the prosecution, finding Miller guilty of murder.

However, Miller's ongoing mental health while behind bars and the fresh expert report appear to have shed new light on the cause of Miller's psychosis and could confirm what his defence was at trial.