The fire was on the ground floor of the three-storey building, behind the stage area.
"It was quite well-contained due to the construction of the building," Peachey said.
It's understood most of the damage was smoke-related.
A police spokeswoman said they were flagged down by a member of the public who noticed smoke coming out of the roof.
Nine fire appliances spent three hours battling the blaze at the Hawke's Bay Opera House. Photo / Adam Codlin She said their role was to assist Fire and Emergency crews.
"We are working with fire to determine the cause."
Last year the government invested $4 million towards the cost of earthquake-proofing the Opera House.
Reconstruction of the building is ongoing.