I have one of these on my post box since all my mail goes to a PO box and I don't actually monitor the street box except for once every few weeks for RTS mail.
Today having checked the mail box I found some junk mail, rang one of the company's to give them a serve and was told that Auspost are exempt from the no junk mail sign even if the mail is very clearly junk mail.
Anyone able to confirm the Auspost exemption, this was an unaddressed real estate leaflet and very clearly junkmail.
At this stage plan B is to simply gaffa tape over the letter box opening.
Cheers
Paul