One of the main diseases that claims the lives of koalas, chlamydia, has been found to have spread through almost two thirds of a colony in southeast Victoria.
Scientists from Monash University made the discovery and are warning that the disease could spread, possibly reducing koala numbers in the process.
They tested the droppings of wild koalas living in South Gippsland and found 61 per cent of samples tested positive for chlamydia.
However koala retrovirus - an AIDS-like virus about which little is known but is another major cause of death for the famous marsupials - was lower than previously thought and present in only 27 per cent of droppings.
Chlamydia, which can cause infertility and blindness, has been blamed for causing dramatic reductions in many koala populations in NSW and Queensland.
Scientists have been working on vaccine to try and prevent its spread.
The Monash University researchers behind the South Gippsland study said further research was needed to help work out strategies to conserve the koala population in the area.
"Given the high prevalence and strain diversity of C. pecorum (chlamydia) throughout the South Gippsland region, and the likelihood of increasing environmental pressures in the future, the incidence and/or severity of overt disease may increase in the region over the coming years," they wrote in a research paper published in the journal Wildlife Research on Wednesday.
"Due to the conservation importance of the diverse remnant koala population in South Gippsland, monitoring of these infections in the region over time is vital."
The study was the first large-scale investigation of the prevalence of chlamydia and retrovirus in South Gippsland's wild koala population.
The disease was found to be more common among female koalas than males, while the rate of retrovirus was similar for both sexes.
Koalas living in large populations in the area were three times more likely to test positive for chlamydia than those in lower density areas.
The South Gippsland koalas are considered by scientists to be genetically unique compared to those in other colonies in Victoria, many of which are home to the ancestors of koalas that were introduced from other areas.
The Monash researchers say that the significant greater genetic diversity among the South Gippsland koalas make them a high conservation priority.