Scaffolding has gone up on the outside of part of the 134-year-old building, as workers from Dooleys Masonry in Oamaru begin work on the roof.
Dunedin City Council city property asset manager John Varney said worn features on parts of the building's Oamaru stone balustrades were being replaced.
The work began earlier this week and was expected to take up to four weeks.
It would cost about $25,000, he said.
The planned maintenance had been timed to fall between the end of the cruise ship season and the onset of winter, he said.
It was not a major job, although working at height made it ''a bit more complicated''.
In total, 18 of the turned, shaped pieces of stonework would be replaced around the top of the building, where it faced the Octagon and also along part of Municipal Lane.
''Over time, the Oamaru stone deteriorates. It weathers ... and we just replace like for like,'' Mr Varney said.
