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About 60 posties are based in Dunedin. Nationally, the move is expected to cost 400 jobs.
The state-owned enterprise has confirmed deliveries will be cut from six days a week to three.
The change will take effect in urban centres in July.
''We will work with staff on the delivery design between April and June.
"Final job numbers in each location will not be confirmed until the end of this process,'' an NZ Post spokeswoman said.
Dunedin man Victor Billot, an organiser of the failed campaign to save the post shop in the Exchange in 2011, said there was a lack of public consultation about this week's cut to a ''major public service''.
The 2011 campaigners were ''ignored and patronised'' by New Zealand Post, which reflected the organisation's ongoing attitude to the public, he said.
''Moving down to a three-day-a-week delivery, that's killing the usefulness of the service. It's making it a worse service, and I wonder if that's ... intentional.''
The rejuvenation of the Exchange showed the post shop should have stayed. It was a ''classic example'' of poor decision-making.
Mr Billot said the many people who still depended on post were being ignored, and the possible social impact had not been analysed.
''I'm actually astounded there's not more organised concern about it.''
Postal Workers' Union advocate Graeme Clarke, of Wellington, believed postal volumes were still higher than in the 1980s and 1990s, despite the decline of recent years.
Asked about the lack of public reaction, Mr Clarke said: ''Those who care don't really have a voice. Posties get stopped by a lot of people who complain about the decline of the service.''
Mr Clarke said the ground was prepared a couple of years ago when next-day across-town delivery was dropped, with mail centres in provincial areas like Dunedin significantly downgraded.
Dunedin postie Karen Sutherland said she was ''quite relaxed'' about the changes, because ''something had to happen''.
However, she believed posties could end up working harder once their numbers were cut.
The New Zealand Post spokeswoman said mail volumes ''cannot be compared'' with those of 30 years ago, because since then businesses started to use the internet to deal with customers.
''This has meant that letter volumes have plummeted over the past decade to 642 million items in the last financial year. Volumes are dropping by about 8% or 60 million items a year, and are forecast to be under 500 million in 2015-16.''