"These terrible crimes were committed in tragic circumstances which ended the lives of the offender's parents," said Justice Peter Johnson in the NSW Supreme Court on Friday.

David Reid, 48, pleaded not guilty to murder but guilty to the manslaughter of his parents Diana, 71, and Graham, 75, at their Sutherland home where he had lived all his life.

The crown accepted the pleas to the lesser charges on the basis he was substantially impaired by an abnormality of the mind.

On August 6, 2019, Reid stabbed his mother as she ate breakfast before going into the bedroom and stabbing his father while he was in and then getting out of bed.

He then rang triple-zero saying "I stabbed my parents and tried to commit suicide".

In jailing him for 10 years and six months with a non-parole period of five years and six months, the judge said there was "no rational explanation for his homicidal attacks".

Reid, who was described as a loner and a "man child", had no prior criminal history or any history of violence.

But experts agreed he was significantly impaired at the time by severe depression and avoidant personality disorder for which he previously had received no treatment.

He had attacked his parents without any triggering event or argument, but had clearly been dwelling on his "distorted and bleak perception" of them, the judge said.

In the lead up to the stabbings, he had posted to social media platforms bleak notes referring to things going bad, feelings of despair and suicide.

After obtaining a Bachelor of Engineering degree, Reid had worked since 1998 as a software engineer for a company producing poker machines.

The quality of his work had been described as "from very good to poor" and at one stage he was sleeping at his desk but improved after receiving warnings

Reid told a psychologist he felt a failure, his job was meaningless and he felt defeated.

He felt his parents had given up on him, but the judge noted there was no evidence they ever neglected him and his feelings were based on his distorted perceptions.

His parents had cared for him and provided accommodation for him throughout his entire life.

There was no rational basis for him to have any grievance towards them, but he stabbed them in circumstances which must have been terrifying for each of them.

Reid's only sibling, Andrew, and his wife Milly provided victim impacts statements revealing the devastating consequences of the loss of the parents and, in a way, of his brother.

"The destructive effect of events such as this are magnified when the offender is a member of the family, so that the inexplicable nature of what has happened is confounding for (the relatives) who are left to grapple with the loss of a generation of the family," the judge said.

It was up to the prison authorities, not the court, to decide whether Reid would be housed in a forensic hospital or somewhere else.

"What is clear, however, from the evidence is that ongoing assessment and treatment of the offender is important and should be undertaken," the judge said.

"These steps are in the public interest and will operate to protect the community as well as assisting the offender."

Australian Associated Press