The Postal Worker's Union says New Zealand Post employees would not be facing such significant job losses if there had been better regulations relating to private mail companies.
The Government is allowing New Zealand Post to cut delivery of standard letters from six days a week to three in urban areas, which could cost up to 1000 jobs.
Chief executive Brian Roche has conceded the changes could lead to a halving of the number of posties in work, but says it will be some time before a definite number of job losses is known.
Postal Worker's Union chairman John Maynard says the union is concerned that if job losses are as high as 1000, there could be a negative impact on the service to customers.
Mr Maynard says one of the major factors is a clause in the Postal Services Act 1997 allowing private companies to collect and deliver mail.
However, he says only New Zealand Post is required to provide a national service at a standard rate, so private companies are cherry picking the most profitable areas.
Mr Maynard says that equates to an unfair subsidy to private companies, which disadvantages New Zealand Post.
He believes private companies have taken 10% of the mail volume, and says if that had not been allowed to happen, the job losses would not be so serious, so quickly.
New Zealand Post disagrees, saying the fundamental driver for change in the postal market is not competition, but declining letter mail volumes spurred by electronic alternatives.
It says it stands by the amended deed it presented to the Government in January and believes the appropriate process from that point was for submissions on the proposal to be made via the Ministry of Business Inovation and Employment to the Government.
Cuts 'unjustified'
The Engineering, Printing and Manufacturing union says the move to slash postal delivery is shocking and unjustified.
EPMU postal sector co-ordinator Joe Gallagher says the decline in postal volumes is the reason the union worked so hard to switch posties to a volume-based payment system.
He says that move means cutting the delivery service from six days to three is unnecessary.
However, New Zealand Post chairman Sir Michael Cullen told Morning Report that unfortunately the rate of decline is accelerating, with the number of letters down by 8% per year.
"We know on an average day ... we miss out every other letter box, we go to just about 50% or so of delivery points each day already."
Sir Michael says New Zealand Post will talk with the union about how exactly the changes will be implemented.
Premium services unchanged
The changes, which will apply from July 2015, won't affect courier post or premium services like Fast Post. The rural delivery service has also been spared cuts, and deliveries there will be at least five days a week.
There will also be changes to the way Postshops run, with more automated services such as self-service kiosks.
Former New Zealand Post chief executive Elmar Toime says the decision to cut the number of delivery days for standard mail was inevitable.
Mr Toime, who now heads a global postal consultancy based in London, told Morning Report delivery flexibility is argued about the world over.
He says long-term, mail delivery will be difficult to sustain as volumes fall and mail is substituted by internet and other forms of communication.
Mr Toime says as the postal business declines, customers can pay more, the Government can subsidise the service or the service levels have to change.