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 9-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.