Interestingly... I think I have answered my dilemma of missing packages, finally! My last four items have not turned up, and eventually tracking says they were 'undeliverable' and they get returned to the mothership.
About 3 years ago, NZ Post changed our postcode, seemingly no one else has caught up with this fact yet. I changed my postcode back to the old one, on Amazon, bought something cheap with the credit they gave me for all the missing packages, and voila! The latest item not only turned up today, but turned up 8 days early!
Maybe others have this issue too?