The application, by former St Bathans resident James Cameron, for a 10-year concession attracted 195 submissions, 190 of them opposing the plans.
A hearing was held in December, with more evidence heard in February. Doc Central Otago manager Mike Tubbs and acting Otago conservator Alan McKenzie announced the decision in a press statement yesterday.
Mr Cameron welcomed it as a "good compromise", while opponents, advised of the decision by the Otago Daily Times, said their initial response was disappointment.
The St Bathans Heritage and Environment Preservation Trust would meet to discuss the matter once it had seen the full decision, a spokeswoman said.
"From the Doc press release, we are pleased to see that the public will have access to a small part of the building, but are disappointed that Doc do not seem to understand the importance of maintaining the building's historic fabric in its entirety, as part of preserving our built heritage for future generations," the trust said in a statement.
"The lack of will to find an appropriate and compatible tenant for the post office is apparent and this being the case, we question whether Doc should be charged with managing such an historic reserve."
The building has been classed a historic reserve since 1981, administered by Doc, and has category 2 status with the New Zealand Historic Places Trust.
Submitters were mainly concerned about the lack of public access to the building's ground floor and changes to historic fixtures and features, and Doc said it had taken those views into account.
Public access would be retained to a dedicated space on the ground floor of the building, containing counters and telephone booths, where the post office activities were once carried out.
The approved concession also included "several reporting requirements" for Mr Cameron, relating to features and fittings.
Mr Cameron, who lives in Christchurch, said although the change to his plans would affect his income from the building, it was "a pretty good outcome".
It had taken three years to get to this point and his next step would be to apply to the Central Otago District Council for planning consent to change the use of the building, he said.
The details of the concession were yet to be finalised.
Central Otago District Council planning team leader Ann Rodgers said the council had not yet received the resource consent application. After it received that application, a decision would be made on whether it should be notified.
The only commercial use of the building interior during the past 30 years was a business run by the late Sharon Hinds between 1995 and 2008.
She operated the building as a postal bureau, also selling gift items relating to the goldfields era.
Doc visitor and historic assets manager Anita Middlemiss said the decision would ensure the building could be "preserved for future generations to use and enjoy" while also allowing for the public to access part of the building.
Under the Conservation Act, concession applicants can apply for a reconsideration of the decision.
Submitters wanting to take the matter further can seek a judicial review in the High Court, in respect to how the decision was made.
