Koala Rescue Queensland (KRQ) responded to a report of one of the furry animals climbing a picnic shelter at Brooloo Park Lookout near Gympie, 175 kilometres (around 100 miles) north of Brisbane, on Wednesday.
But on closer inspection it was found to be dead and screwed in place.
"This poor koala has been screwed to the pole with building screws," it said on Facebook.
"He is deceased, but whether or not he was when cruelly attached to the structure is unknown."
KRQ president Murray Chambers said he thought he was hearing things when told about the incident.
"We've had koalas shot a few times. We've had them deliberately run over, but this is the first time some nutcase has actually screwed a koala to a building," he told broadcaster ABC.
"You know, I mean, what's wrong with society? Just makes you feel sick in the guts."
The gruesome find sparked outrage on the KRQ Facebook page, with hundreds of comments slamming the "sickening" act.
"Hard to imagine that anyone could stoop so low," said Tim Siggs, while Elly Bunting added: "Absolutely horrendous. I really hope these grubs are caught and are dealt with and NOT with a slap on the hand."
The much-loved koala has been under increasing threat across Australia in recent decades, particularly from habitat loss, disease, dog attacks and bushfires.
A 2012 national count placed total numbers at 330,000, though their tree-top habitat makes accurate assessment difficult.
It is not the first time a koala has been mistreated.
In November, one was found dead in Victoria state with both its ears cut off, the latest in a spate of animal mutilations around the town of Warrnambool.
Police said at the time it followed a series of kangaroo and wallaby mutilations in the area.
Earlier last year, a kangaroo was found shot dead, dressed in leopard-print, and tied to a chair holding a bottle of ouzo, also in Victoria state.
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