Country Fire Authority Chief Officer Jason Heffernan said: "Tomorrow is a very, very dire bushfire day in the state of Victoria."
The fires will cause around 450 school closures and the cancellation of many regional train services.
The Longwood fire covers more than 61,776 acres, while the Walwa fire is 24,710 acres and has created its own weather system, with a pyrocumulonimbus cloud causing lightning and thunder.
The fires have destroyed at least two structures and are expected to spread further on as the wind and heat pick up.
Total fire bans were issued in several districts, with a total fire ban expected will be imposed across the whole state on Friday.
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