HQ Plantations, owners of a large softwood timber plantation at Tuan, said it suffered tree losses worth hundreds of thousands of dollars in the blaze.
HQ Plantations fire management officer Leigh Kleinschmidt said the fire had damaged more than 1000ha including 400ha of plantation.
"The fire investigation is still to be presented in a full report but the initial ignition point has been determined," Mr Kleinschmidt said.
Authorities are appealing for information.
Phone Crime Stoppers on 1800 333 000.