New Zealand Post is ditching its FastPost service from January 1.

Customers will instead have to courier letters that would usually be sent via FastPost for priority mail.

"Only 1 per cent of all mail is sent using this service and during the last year it has declined by 23 per cent," a spokeswoman said in statement on Wednesday.

NZ Post said letter volumes continued to drop, at a rate of approximately 60 million items a year, and the change was necessary.

Business customers posting between Auckland, Wellington and Christchurch will be able to use a new "priority mail" service.

Standard Post won't be affected, NZ Post said.

The decision to wind up FastPost signals the end of an era, but also reflects the reality of today's postal services, the E tu union says, and the change could mean job losses.

However, the union said NZ Post had told it that it intended to redeploy affected posties wherever possible, as business was booming in parcels and logistics.

""FastPost business dropped by 23 per cent over the past year. On the other hand, demand has surged for parcel deliveries for customers such as The Warehouse, My Food Bag and Amazon," said the union's Joe Gallagher.

"While this means change for many of our 500 postie members, there will be opportunities within the delivery area at NZ Post."