Posting a letter will cost 10 cents more from October.
New Zealand Post says the 20% jump from 50c to 60c for a standard-size letter is necessary to ensure the service remains viable and to pass on the rise in GST.
A standard-size letter sent by FastPost will rise by 20c.
New Zealand Post says every letter size will cost more to send domestically from 1 October. International mail rates will also increase.
Postal services chief executive Peter Fenton says it is delivering 20%less mail than three years ago, although the number of delivery addresses has risen by 160,000.
The last general rise in postage was three years ago when the cost of sending a standard letter increased from 45c to 50c.