fixated person
The concept of the fixated person as a person worthy of scrutiny by the police was one which arose from the Lindt Café siege. At the time everyone assumed that the gunman was another person who had adopted terrorist tactics in support of ISIS or Al Quaeda but it emerged that he had no links with any terrorist organisation. This was a personal terrorist attack motivated by various grievances and possibly mental health issues.
So the police needed to define who this kind of person was and what rights the police had to intervene in their activities. The fixated person is someone obsessed in some way, whether with a public figure or celebrity, or someone in their circle, and possibly motivated by a grievance or sense of a personal cause for which they think they are fighting. We now have a Fixated Persons Investigation Unit (FPIU) in NSW which can investigate such people. There is ongoing discussion around Australia with professionals in psychiatry and law to establish the regulations relating to such investigations. Recently NSW politician John Barilaro was stalked and harassed by a person deemed by the police to fall into this category.