Maybe check your address on their system. (if the address label on the envelope is missing something)
When I first registered at the beginning, they didn't have a place for "State codes" because their "State" box was for American states, so I put the QLD next to the suburb in the Suburb box (so it prints out on the address label between suburb and postcode).
Ever since they changed their system to allow for Australian states, I keep trying to change my address in their system, but it keeps including the QLD as part of the Suburb, so it ends up having it there twice.
The other thing to note is that my address labels only ever have "AU" on them instead of "Australia", so if the same is happening to you, maybe a combination of the two is causing the problem.
(sometimes I get the feeling that the newsletters are sent in bulk somewhere, and then forwarded individually within Australia... if that is cheaper - if that is true, maybe yours keeps getting bulk-sent to the wrong location each time... maybe ask them if they do that, and if they do, ask them to fix it)