That's so unprofessional.
I think it is time to call them up about the earlier two, now that it is beyond 6 months... and then check up on the recent two while you have their attention so that you don't have to wait another six months. (use the phone number in the first post)
Maybe they only send shipments to Australia every 6 months - maybe ask how often they send stock to Australia, and if it is less than the time since those issues were released here, I'd be wondering why they haven't sent the missing issues within that time, or in a timely matter.