Thomas Dillan Stone, 27, pleaded guilty in late 2018 to the murder of Robert James Mitchell on the grounds of Morisset Hospital, in Lake Macquarie, in February 2017.

In a triple-zero phone call played at his NSW Supreme Court sentence hearing on Friday, Stone is heard telling the operator he has "chucked" away the weapon and that his victim - "just another patient" - has left to get help.

"Does he (Mr Mitchell) need an ambulance?" the female operator asks, to which Stone replies: "Probably."

The operator then asked: "Why did you stab him?"

Stone said: "Felt like it."

The victim's mother, Gay Crooks, told the court she thinks of her son first thing every morning and before she goes to sleep.

"Knowing the fact that my son, Rob, had desperately made every effort to get to safety and to get help for himself," she said, reading from her victim impact statement.

"The distance, the pathway ... with such life-threatening injuries. This truly causes me the most heartbreaking pain."

Ms Crooks says she remains unable to sign her son's memorial attendance book and grieves for the experiences he'll never have.

A crown witness, forensic psychiatrist Dr Jonathon Adams, said there was clear evidence Stone "transitioned down the levels of security" at the hospital where he'd once been in high support.

At the time of the murder, he was housed in low-support cottages where he cooked his own meals, attended groups and meetings, and was granted unsupervised leave from the hospital campus.

"My view, in summary, is that I do not illicit any clear evidence to conclude there was a nexus between Mr Stone's symptoms of schizophrenia and the offence," Dr Adams said.

The psychiatrist said he believed Stone would "manifest a significant violence risk" for a number of years.

Stone's sentence hearing before Acting Justice Peter Hidden continues.

Australian Associated Press