Fire and Emergency New Zealand fire investigator Murray Milne-Maresca said the fire, which started about 6pm on Sunday while the boat was in Bluff port, had been identified as having been caused by an electrical event around the battery area in the engine.
MV Cindy Hardy is operated by Cruise Fiordland and can carry up to 10 passengers.
The fire was brought under control by 8pm, but crews remained there past midnight to dampen down any hot spots.
MV Cindy Hardy was built in 1963 in Tasmania of Australian hardwood.
It has had a complete refit, including private en suites in each cabin, a new galley and passenger lounges.
Fenz said the damage was considered extensive and it was working with the owner and insurance companies.
Environment Southland said anytime there was a fire on a boat there was a concern that it might discharge contaminants into the marine area or sink.
The council’s harbourmaster spent time on site checking there were no discharges of fuel. The water used to extinguish the fire was pumped from the vessel, so there was no discharge of this either.
The boat was lifted out of the water for the fire investigators and insurers to do their assessments.
