TV3 reported on this the other night, but I can't find a link to the video clip. Apparently from July 1st, 2008 postcodes must be used when sending bulk mail to receive discounts. This is to force bulk mailers to move over to the new system which can be read easier by NZ Post.
According to the news clip residentual mail may also be affected by this change over. Mail can be lost in the mail system for months if the wrong/old postcode is used. Examples were given where there are ~18 Beach Rd's in Auckland. Using the correct postcode allows mail sorters to send your letter to the right one.