Police did not confirm media reports that the man, Mert Ney, had absconded from a psychiatric unit two days earlier, but said he was known to NSW mental health services.

NSW Commissioner Mick Fuller said the man had material in his possession that suggested he had "ideologies related to terrorism" but no apparent links to terrorist organisations.

The incident comes two weeks after the NSW opposition health spokesperson called for an urgent review of the mental health system following the case of a woman arrested for allegedly cutting off her mother's head.

The woman had appeared in court several times over the previous five years but had applied for an exemption under the Mental Health Act, raising serious questions about the level of mental healthcare she received.

In July, the family of a Carlingford man who stabbed his five-year-old son in the belief he was the devil said they had tried to have him hospitalised two days before the killing but were told there were no beds available.

A veteran psychiatrist working in one of Sydney's largest locked psychiatric wards cautioned the public against extrapolating damaging judgments linking violence to psychiatric patients.

"We don't know what drove this alleged perpetrator to commit these abhorrent acts, and the fact is people with serious mental illness are far more likely to be the victims of violence than the aggressors," she said.

A senior psychiatrist at a major NSW hospital said patients absconding from psychiatric units was fairly common and acts of violence by these patients was "incredibly rare".

"Violence by people with mental illness is very rare overall, so the risk of violence by a patient who has absconded is even less likely," the psychiatrist said.

He said common ways of preventing patients absconding was to install more doors and higher walls, which had the unfortunate side effects of creating an unpleasant and untherapeutic environment.

A psychiatrist working with psychiatric inpatients with complex conditions said hospitals were required to treat patients in the least restrictive way possible.

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"They're not supermax prisons, but we do have some precautions against absconding, for instance pretty much every psychiatric ward in NSW is now a locked ward."

He said the majority of public mental health inpatients have some form of leave that allows them to exit the unit every day, many unaccompanied.

"It is fair to say that at any given time most psychiatric units in most weeks will have a patient who are absent without leave and there will be some level of anxiety about it, usually that they won't come back or get themselves into strife," the psychiatrist said.

"If patients want to abscond they find a way. Some are very athletic and some are very clever and some regard it as a challenge."

But he said there were additional restrictions for people assessed to be potentially dangerous to others.

The most common adverse outcomes from absconding were minor, for instance, patient turning up drunk.

"A rare outcome is suicide and homicide is very, very rare but highly catastrophic," he said.

A large body of research shows people with mental illness are no more violent than the general public.

Mentally ill people have a greater risk of being victims of violence, particularly those with complex conditions and psychosis, and have a higher risk of homicide, suicide and self-harm.

If you or someone you know is experiencing problems, call Lifeline on 13 11 14 or go to lifeline.org.au, or call beyondblue on 1300 224 636.