I live in Potts Point and our external letterboxes are rifled about once a month. I have a PO Box at the PO but most residents do not.
As a long-standing Exec Comm member I have tried on and off for more than 10 years to get Australia Post to allow us to shift our letterboxes inside our front gate. They flat-out refuse to accept any more keys/fobs which would be needed for the postie to gain access to sort the mail.
They cited incidences of posties having their bicycles, motorbikes, carts etc damaged or stolen whilst inside premises attending to post deliveries off the street.
So encouraging residents to shift letterboxes inside is not going to work, at least from my experience.