It has also accused NZ Post of deliberately "running down" its mail service.
Yesterday, NZ Post chief executive David Walsh said the service would move to deliver mail alongside courier parcels.
"This decision means that there will be significant job losses in existing mail delivery, processing and support roles," he said.
Over time, having two separate delivery networks would no longer be commercially viable, Mr Walsh said.
Postal Workers Union of Aotearoa co-president John Maynard said although the news was not unexpected, both the timing and the way the information was delivered left a lot to be desired.
"Once again, NZ Post is making public statements which appear to be designed to undermine people’s confidence in the mail service.
"This is the third time the company has done this — about five years ago, they announced ‘every-second-day delivery’, last year they’ve talked about axing about 700 jobs and [yesterday] they’ve made this announcement.
"It’s been a feeling for years among posties that the company is deliberately running down its mail service."
Twenty years ago, New Zealanders sent more than 1 billion mail items a year — but this has decreased dramatically to about 220 million.
NZ Post predicted mail would further decrease to about 120 million items a year by 2028.
The postal workers’ union accepted there had been major declines in mail volumes, Mr Maynard said.
However, the union believed there could have been different approaches rather than simply cutting hundreds of jobs and employing contractors.
He said posties on electric cargo bikes and motorcycles were a more efficient, environmentally friendly, socially desirable, safer and good employer alternative to mail delivery from vans.
"We understand the company needs to adjust, but it’s hard to reassure posties to the contrary when the company makes statements two years’ ahead of time.
"We’re talking to our lawyers about this, and we have not ruled out taking action.
"We believe that the company may be in breach of the State-Owned Enterprises Act, the Employment Relations Act and the collective agreement with the postal workers union."
NZ Post planned to implement the changes over the next few years, which meant moving to a contractors model.
Mr Maynard said the contractors’ conditions were undesirable.
"It’s a third-class group of workers: they don’t get annual leave, sick leave or bereavement leave.
"But neither do they have the rights of genuine independent contractors.
"The control the company has over the contractors is quite shocking."
In a statement, Mr Walsh said: "Since this is a long-term plan, no employees are directly affected by the move to one delivery network right now and our focus is on supporting our people with this change".
"For customers — this means your mail and parcels will eventually be delivered by one person, rather than two separate deliveries made by a postie and a courier.
"There is nothing that New Zealanders need to do differently."
