Extremely prepared.
Every day, the ATP are arresting people at dawn raids and taking them into custody. There is a "media black out" on everything related to anti terror operations, with only some stories being released. We are much more prepared than we let on.
Obviously, like some have said, there is nothing to prevent a nut case grabbing a weapon then going on a rampage. No one can prepare for that.
With the known suspects that carry out attacks, even though they may be monitored, they may be elusive or cover their tracks well. Also, if they were arrested, then no evidence was found etc, then they may be open for human rights violations, being sued etc?