No one can know the mind of Sydney shopping mall killer Joel Cauchi, but psychiatrists say one underlying cause of his rampage is evident: he had schizophrenia, stopped his medication and fell out of treatment.

People have searched for a fathomable motive since the April 13 knife attack at Bondi Junction, in which five women and a male security guard were stabbed to death and another dozen wounded, including a nine-month-old girl.

Cauchi's parents have said their son was diagnosed with schizophrenia at the age of 17 and was successfully treated for about 18 years.

A serious mental disorder, schizophrenia can cause hallucinations, delusions and disordered behaviour. It requires lifelong treatment.

New South Wales police commissioner Karen Webb said it was "obvious" to her and detectives that the 40-year-old Cauchi had targeted women and avoided men, sparking a media debate about misogyny in Australia.

Prime Minister Anthony Albanese described the gender breakdown among the victims as "concerning" and vowed to do more to combat violence against women, citing a toll of one woman dying at the hands of a man they knew every week.

"But we will never know what was in the mind of the perpetrator of these acts," said professor Ian Hickie, co-director of health and policy at the University of Sydney's Brain and Mind Centre.

"Ordinary people are trying to impose a rational explanation," he told AFP. "The most obvious one is the irrational mind of the perpetrator."

Recurrence of the mental illness cannot by itself necessarily explain the violence against other people, which is "extremely rare" in such cases, Hickie said.

"Often these things are complicated by other factors; drug use, disconnection, social isolation, homelessness."

No two psychotic people have the same thoughts, which are shaped by each person's idiosyncratic, irrational perceptions of the world, Hickie said.

Cauchi may have attacked women simply because men were better able to defend themselves -- like Frenchman Damien Guerot, hailed a hero for fending off the assailant with a metal post, he said.

- 'Acutely psychotic' -

"The wider issue of domestic violence and the number of women who are harmed or killed by men who do not have any mental illness in our country is a national problem. I don't think this is a manifestation of that problem," Hickie said.

"The social factors that matter here are homelessness and isolation, and the stigmatisation of treatments for mental illness."

Cauchi's parents say he gradually came off his medication over a number of years in consultation with a doctor because he felt he was doing well. He moved from the family home in the Queensland city of Toowoomba to the state capital Brisbane, and then recently travelled to Sydney.

He had been living in a vehicle and hostels since leaving home and was only in sporadic contact with his family via text messages.

He appeared not to realise he was becoming sick and "became homeless, completely disconnected from any sources of support, and ended up in this very acutely psychotic state", said Patrick McGorry, professor of youth mental health at the University of Melbourne.

His behaviour was "completely disorganised or delusionally based", said McGorry, a former president of the Schizophrenia International Research Society.

Attempts to ascribe Cauchi's actions to misogyny were "completely off point".

"It's purely a case of untreated or poorly treated mental illness," he said.

It highlighted that Australia's mental health system was "completely inadequate" to the task of ensuring patients such as Cauchi received continuing care.

"It is true that he wanted to move cities but in that case the healthcare should have been set up for him at his destination," McGorry said.

If patients with schizophrenia stop medication, there is an 80 percent-plus chance of the disease returning, he told AFP.

"And when it comes back, it is likely that the person will not recognise that it's returning and won't seek help."

- Nowhere to go -

Carolyn Nikoloski, chief executive of Australia's peak advocacy group Mental Health Australia, said there was a gap in support for people with complex care needs.

People were often turned away from hospital emergency departments because their illness was not judged to be serious enough at the time, she told AFP.

"That's a common experience, and there is nowhere else for them to go," Nikoloski said.

"We know that overall mental health expenditure does not meet the burden of disease, and it has declined over time."

The health system was unable to catch people who fell between the cracks, said professor Anthony Harris, head of psychiatry at the Sydney Medical School with a special interest in psychosis.

"The real issue here is that this man is diagnosed with schizophrenia -- that is one of the most severe mental illnesses you can have -- but he just drops out of care and drops out of community," he said.

"If you had cancer, if you had a severe physical illness, there's this whole system of follow-up care," Harris said. But with a severe mental illness, "nobody seems to blink an eye".