Rae Campbell said someone dropped Kao over the fence at her home in the Adelaide suburb of Trott Park at about 10 a.m., The (Adelaide) Advertiser reported.
Campbell said she heard a car driving away but did not really want to know who had taken Kao.
"I didn't see who it was, I don't care about that. I'm just thankful that they have brought her back," Campbell said.
Campbell, a member of Fauna Rescue South Australia, took Kao in four years ago when her mother was hit by a car and killed. The koala became something of a celebrity last year when Prince Charles was photographed holding her at Government House in Adelaide while his wife, Camilla, duchess of Cornwall, held another koala, Matilda.
Kao appears to be in good shape except for a sore paw because of a rough landing, Campbell said. She plans to have the koala checked by a vet.
Koalas are herbivorous marsupials somewhat resembling small bears. They are found in most of Australia and have become one of the country's signature animals, along with the kangaroo.