Koalas living on an island off the coast of South Australia (SA) could be the last in the country entirely free of chlamydia, researchers have found.

According to scientists from the University of Adelaide, Kangaroo Island could be a lifeline for koala populations, most of which have been devastated by the disease.

Every large koala population on the Australian mainland is affected by the bacterial infection, which causes blindness, infertility and death.

By comparison, those on Kangaroo Island where koalas were so prominent in the late 1990s that they were sterilized in large numbers are free from the disease.

"The impact of chlamydia on populations of koalas in Queensland and New South Wales is devastating, with high levels of severe disease and death, and common infertility," the University of Adelaide's Jessica Fabijan said in a media release on Tuesday.

"This last large, isolated chlamydia-free population holds significant importance as insurance for the future of the species. We may need our Kangaroo Island koalas to re-populate other declining populations."

The team captured 170 koalas from Kangaroo Island and 75 from mainland SA and tested them for the disease.

They found that 46.7 percent of those from the mainland tested positive while none of those from the island showed any sign of the disease. Of 13,000 past examinations of Kangaroo Island koalas, there have been no definitive cases.

"There are current trials of a chlamydial vaccination in northern koalas to protect them from infection, but it is an uphill battle," Fabijan said.

"Our chlamydia free Kangaroo Island koalas could in the future provide a safeguard against this serious threat to this iconic species."