Tasmania's Tungatinah Power Station, which supplies five per cent of the state's energy, went up in flames on the night of September 2.
The cause of the fire was accidental, the Tasmania Fire Service said on Thursday.
"Preliminary investigation findings have identified several potential ignition sources, which will be subject to further investigation," fire service deputy chief officer Matt Lowe said.
"However, the fire has originated on the assembly bay floor, away from the main turbines."
The investigation had been complex due to the size of the station and significant structural damage it suffered, as well as severe weather in the area.
Tungatinah was one of 10 major power stations built as part of a large-scale hydro-power scheme in Tasmania and was progressively commissioned between 1953 and 1956.
Hydro Tasmania chief executive Rachel Watson said it was too early to tell how long it would take to rebuild the station and bring it back online, with a timeline depending on the condition of its turbines.
Workers and contractors spent the weekend removing debris, stabilising the structure and restoring communications and water to support recovery efforts.
"The building has sustained significant damage and unfortunately will need to be demolished," Ms Watson said.
"We won't be able to confirm the state of the generating equipment until we can get full and safe access to that part of the station."
It had been an incredibly difficult time for staff who had a close connection to the remote station in the Central Highlands, Ms Watson said.
The full investigation is expected to take several months, the fire service says.
About 300 customers temporarily lost power as a result of the blaze, which took 25 emergency crews and 45 personnel hours to bring under control.
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