The company plans to lay off 700 posties over the next five years and pass their duties to contracted courier drivers.
NZ Post last year said it would "trial a whole range of things" to figure out what worked best in response to declining mail volumes.
Twenty years ago, New Zealanders sent more than one billion mail items a year - but this has decreased dramatically to around 220 million.
NZ Post predicted mail would further decrease to about 120 million items a year by 2028.
The Postal Workers Union made a counter proposal in February and expects a response by the end of this month.
Union co-president John Maynard said forcing couriers to carry letters in their vans would slow everything down.
The electric bikes used by posties were more nimble and environmentally friendly than vans, he said.
"Van delivery will be slowed down because it's much quicker to go along the footpath on an electric bike. We're saying that the posties holding onto mail and small packets and parcels is a more efficient way of delivering it than small packets and parcels and mail being delivered by diesel and petrol powered vans."
Parking would be a big issue for courier drivers if the plan went ahead, Maynard said.
NZ Post's plan to replace postal workers with contractors violated the State Owned Enterprises Act, Maynard said.
As a state-owned enterprise, NZ Post was required to provide good and fair working conditions, he said.
Maynard said he believed replacing employees with underpaid contractors constituted a breach of the act.
Couriers contracted by NZ Post did not have the rights of employees, but they were expected to work similar hours, he said.
Earlier this week Maynard told RNZ's First Up that the Postal Workers Union was putting a counter proposal to NZ Post to keep posties.
People holding on to their letterboxes could be viewed as a statement of support for the current postal service, he said.
