New Zealand Post says a $10,000 nationwide competition to remind people to use postcodes is not aimed at cutting jobs.
The Postal Workers Union says the more postcodes are used, the more mail can be sorted by machines. It says some postal workers are worried that the competiton will lead to fewer jobs.
Union spokesperson John Maynard says while workers might not be laid off, there will be fewer jobs available in the long term.
However, New Zealand Post says businesses are already using postcodes and the campaign is to remind the general public to use them as well.
Marketing general manager Graham Smith says postcode use has risen from about 15% at the start of the year to about 50%.