If a buyer contacts you re blocks due to address go to Accounts> site preferences>buyer requirements>buyer requirements activity log. This wil show you log of blocked buyers you can manually overrides buyers who primary address is blocked.

this is worth ever5yone checking as you may not know it is happening

also if on your listings you have countries you dont ship to and you have Oceania checked, it can block Aus sellers. I had a couple of listings were this had somehow been selected.