The snail mail might take even longer to arrive in New Zealand.

New Zealand Post is considering reducing its delivery services from six to three days a week and managing a lower staff count in response to an "irreversible" downturn in postal revenue.

A letter from NZ Post chairman Michael Cullen to State Owned Enterprises minister Tony Ryall shows the board is seeking urgent changes to a 1998 deed requiring the Post to run a six-day-a-week postal service for 95 per cent of New Zealanders.

In his letter to Mr Ryall, Mr Cullen said launching new products to counter dropping postal revenue or further cutting cost were no longer realistic for the state-owned agency.

Mr Cullen said the board had exhausted all "short term fixes" and it is now time for a fundamental change to operations.

NZ Post chief executive Brian Roche told Radio New Zealand the Post was considering cutting deliveries to three days a week, considering people have been increasingly using email communication anyway.

"That's very sad but it's inescapable," said Mr Roche, referring to the number of staff that would be affected by the changes.

NZ Post had suffered from the fastest ever decline in total mail volumes in 2012, and it would start losing money from 2015 if no changes were made, the New Zealand Herald reported.

"There is no turning back from this decline - every developed country in the world is facing year-on-year falls in mail volumes of five per cent... If New Zealand Post did nothing to change its processing and delivery systems, those postal losses would start at $10 million, balloon to over $20 million the next year and keep on growing," Mr Roche explained.