The Department of Conservation advertised its intention this month to grant a 10-year concession for the 102-year-old building to be developed and operated as commercial accommodation.
Public submissions are being sought on the proposal, with the closing date December 1.
Last month the Otago Conservation Board confirmed its support for the concession, saying the building would benefit from use and careful renovation that retained its historic values.
It has been classed a historic reserve since 1981, administered by Doc, and has category 2 status with the New Zealand Historic Places Trust as "a place of historical or cultural heritage significance or value."
St Bathans resident Keith Hinds and the St Bathans Heritage and Environment Preservation Trust will be objecting to the concession.
Mr Hinds, one of four permanent residents in the village, said the plans for the building were "a travesty".
His late wife, Sharon Hinds, operated the building as a postal bureau for 13 years, also selling Victoriana and books relating to the goldfields era, attracting "more than 100,000 visitors".
He was concerned about the plans to alter the building interior and also at the possibility the public could be denied access.
The concession applicant, Cameron Accommodation, of Christchurch, sought permission to operate a retail operation on the ground floor along with post office facilities and to modify the building to use as commercial accommodation.
The building would provide accommodation for up to seven people.
James Cameron, of Cameron Accommodation, responded to an advertisement by Doc in 2008 for expressions of interest in using the building.
In his application, he said he had undertaken major historic projects here and in England that involved working "sympathetically" with the historic fabric.
The chairman of the St Bathans trust, Rob Duffy, of Dunedin, described plans to alter the interior as "akin to vandalism".
"It was built in 1909 and its fittings are original and the interior is pretty much untouched since then.
"It's a very distinctive and important building on the streetscape and recognised as such by artists and photographers."
His family owned the post office for 10 years, using it as a holiday home, before selling it to Lands and Survey in 1981.
"The understanding was that the building would be preserved for future generations and would remain open to the public," Mr Duffy said.
There was "widespread opposition" to the concession proposal, he said.
Central Otago Doc manager Mike Tubbs declined to comment on the opposition but pointed out the public had been invited to air its views on the proposal. If submitters wanted to be heard, a hearing was likely to take place before December 14.
