Fire and Emergency New Zealand (Fenz) said it was called to the incident at Ancient Briton Hotel, in Naseby, Central Otago, about 1am.
Fire crews from Naseby, Ranfurly, Omakau and a command unit from Alexandra attended.
The back of the hotel was "well involved with fire" when the first crews arrived. It took about two hours to put the fire out and there were no longer any crews on site, a spokesman said.
The building was occupied when the fire started, but everyone got out and no one was injured.
The fire was contained to the back of the hotel, which received "extensive damage", the spokesman said. There was smoke and water damage to the rest of the building.
A reporter at the scene this morning said the hotel was cordoned off and damaged contents were being removed from the building.
There was not a lot of damage visible to the exterior of the building but inside was significant damage, he said.
It was not yet known how the fire had started, the spokesman said.
A Fenz investigator was on his way to Naseby.
The 1863 hotel was the first hotel built in Naseby during the town's gold rush.
