Police and firefighters were called to a blaze at Karnet Prison Farm in Keysbrook overnight.
The fire started in a mechanical workshop at the low-security facility around 10pm.
It took fire crews over an hour to extinguish the blaze.
All inmates and prison staff have been accounted for and police say there are no suspicious circumstances.
Peel Thunder cruise into 2026 WAFL grand final
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Two teens cautioned, one charged over brutal Falcon attack
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Nearly 100 new rentals to be built in Mandurah
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Blue swimmer crabs off limits during season closure
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Fire crews on scene of Meadow Springs house fire
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Perth's southern suburbs declared One Nation's 'heartland' after Secret Harbour by-election victory
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Construction contract awarded for Kwinana Freeway upgrade
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Port Kennedy humpback whale buried at landfill facility
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Secret Harbour by-election: Where to vote on August 29
Thousands of Secret Harbour electors will cast their vote in this Saturday’s by-election.