Russell Te-Rangi Walker, 30, was found not guilty last month of attempted murder over an incident in which he plunged a large knife deep into his victim's chest, narrowly missing the heart.

Justice David Mossop said in reaching that verdict that he was not satisfied Walker had tried to kill the victim, who had been visiting the Bonython home of Walker's "intermittent" partner for a meal in June 2019.

The judge also found Walker not guilty of an alternative charge of intentionally inflicting grievous bodily harm, this time by reason of mental impairment.

Such a verdict meant Justice Mossop was then required to indicate the sentence he would have imposed had the 30-year-old not been mentally impaired, dictating the maximum period of time Walker can be kept in custody for treatment.

In a judgment published on Wednesday evening, Justice Mossop said Walker had carried out an "unprovoked" attack.

"The evidence at trial indicated that the victim suffered a 15-centimetre wound to his chest and that without prompt and effective medical treatment he would have died," the judge said.

"... it is only fortuitous that the attack did not cause the victim's death."

The judge also referred to a victim impact statement that was read to the court last month.

In his statement, the victim expressed doubt as to whether he would ever fully recover and described having ongoing, "very frightening" flashbacks to the attack.

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"The long-term psychological consequences of the offending are of the type that would be expected from such a grievous, life-threatening incident," Justice Mossop said.

The judge also addressed Walker's "significant" criminal history, which includes a conviction for reckless wounding over an incident that involved him "acting erratically and thrusting a knife in the face of a stranger, causing a wound underneath the person's left eye and two relatively minor cuts to his throat".

"Because of his history of offending and poor control of his symptoms of schizophrenia, protection of the community must be a very significant sentencing consideration, even in circumstances where moral culpability for the offending is lacking," Justice Mossop said.

"His prospects of rehabilitation appear to be largely linked with his capacity to control his symptoms of schizophrenia and the potential for such long-term rehabilitation must be considered to be guarded at best.

"That is because of his history of drug use and the difficulty of maintaining an appropriate medication regime outside a detention environment."

The judge ultimately said that if Walker had been found guilty of the intentionally inflicting grievous bodily harm charge, he would have imposed a sentence of imprisonment for six years.

The sentence would have been backdated to the date of the incident in June 2019, when Walker was taken into custody.

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