Federated Farmers says a reduction in the number of mailing days per week to rural areas is inevitable.
In the past decade New Zealand Post's mail volumes have dropped by 23% and are forecast to decline rapidly in the next five years.
The organisation is currently seeking amendments to an agreement it has with the government about its basic service levels.
Federated Farmers president Bruce Wills says he has had preliminary discussions with New Zealand Post about the current six day delivery service.
He says that some rural areas don't have internet coverage so it would be tough if mail volumes were cut back from 6 days.
However he says economics dictates that the current six day delivery service is not sustainable.
He does not envisage the service being reduced to fewer than three deliveries a week.
John Tulloch from New Zealand Post says a range of options are being considered, including the delivery frequency of standard letter mail.
"We have an irreversible and significant decline in standard letter mail but at the same time we've got a really excellent growth in parcel volumes. So really it's about having a network for the future that recognises where the demand lies."
Mr Tulloch says any change to the frequency of mail delivery is years away.
He hopes public submissions on New Zealand Post's proposals will be possible later this year.