The cost of sending a letter within New Zealand by FastPost is going to increase by 50 cents, outraging the Postal Workers' Union.
New Zealand Post said the price of a stamp on a medium-sized letter would rise 27 percent - from $1.80 to $2.30 - from July. Large and oversized letters would increase by 50 cents.
The rural delivery surcharge on ParcelPost would increase by 50 cents. There would be rises in international and business mail costs, it said.
NZ Post said it decided to increase the charges after its annual price review.
It said the number of letters was falling by about 60 million items a year, while costs were increasing.
Postal Workers' Union southern president John Maynard said the price increase should be scrapped.
The state-owned enterprise should instead focus on getting letters sent on time, he said.
Mr Maynard said people should be able to expect FastPost to arrive the next day.