NZ Post chief executive Brian Roche told Radio New Zealand the agency was considering cutting deliveries to three days a week.
The service was unsustainable as people increasingly used email for immediate communication, he said.
"That's very sad but it's inescapable."
Dropping the number of delivery days would see cuts in staff numbers, Mr Roche said.
"That's, again, it's incredibly sad but it's unavoidable."
An NZ Post statement said mail volumes were expected to drop about 40 per cent to just over 600 million a year in 2018.
The agency had endured the fastest ever decline in total mail volumes this year and would start losing money from 2015 if no changes were made, it said.
"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."
Profits from Kiwibank and growing parcel delivery services were offsetting revenue losses from traditional post, an NZ Post spokesman said.
However, they could not be used to offset postal losses without degrading the business and cutting off investment opportunities, he said.