New Zealand Post is set to cut 500 jobs over the next three months in an effort to save the company $50 million.
The company has confirmed the decision, with E tu union's Joe Gallagher calling it an "aggressive timeline" for the cuts.
The job losses will mainly be at the managerial level, including specialist roles, and mainly from Auckland, Wellington and Christchurch.
In a statement NZ Post says it has entered consultation, and the changes have been "widely signalled".
"This process forms the second phase of New Zealand Post's five-year transformation plan, and follows a number of significant changes that have been made to frontline teams, including processing optimisation, alternate day delivery and integration of the New Zealand Post and Courier businesses."
The 500 job losses form part of the larger 1500-2000 job losses indicated in late 2013 when the five-year plan was announced.
The company says the change will get rid of layers of management, to let it "act on decisions faster and better support our frontline teams working with customers".
"It will also simplify how the organisation is set up, giving more decision-making power to regional and local teams and remove duplication."
It expects the process to be completed by July.
Labour leader Andrew Little has criticised the Government, saying instead of watching the major changes, it should be working to ensure a transition for employees "instead of leaving them on the scrapheap".
"There is no plan for the future from National. NZ Post workers are in a rapidly changing industry. The Government should be actively looking at ways to encourage new sectors and innovative businesses to create new jobs for these people," he says.
"We need to get ahead of the game to prevent wholesale job losses. There's no better example of this than NZ Post."
NZ Post is still facing a major decline in revenue as people send 60 million fewer letters each year.
It has recently started to move into parcel delivery, with the growth of online shopping.