People will have to pay 20 cents more when sending a standard letter within New Zealand from the start of July.
NZ Post announced this week that from July 1, the cost of sending a standard, medium-sized letter would increase from $1 to $1.20.
It said it was making the change, along with other changes to postal products, following an annual price review.
The price of sending a large letter would increase from $2 to $2.40 and an oversize letter would be $3.60 instead of $3.
There would also be product and price changes for business customers who use NZ Post's Bulk Mail products.
Chief marketing officer Bryan Dobson said to provide a commercially sustainable letters service, NZ Post had to review and adjust prices.
"We acknowledge this represents a change, and we have worked very hard to reduce operational costs to minimise impact on customers who value and depend on this service."
He said careful consideration had been taken in making the pricing decisions, "however, given the letter decline challenges NZ Post faces, they are necessary".
"Our postal pricing will now better reflect the cost to process and deliver items, and will help ensure our letters business is in good shape for the future."
Prices would increase for all Standard Post letter sizes and Bulk Mail as well as domestic parcels, courier, international inbound and international outbound letters and parcels.
"These changes will assist NZ Post to manage the significant loss of revenue from fewer letters being sent and still maintain a network of more than 880 postal outlets and 1.98 million delivery points," NZ Post said in a statement.
For a full list of price changes visit the NZ Post website.