They are some of the healthiest koalas left on Sydney's urban fringe, but environment groups fear their local natives are coming under threat.
There are about 300 koalas living in the Macarthur bushland in western Sydney — the largest population of koalas in the Sydney basin.
In the past 20 years the colony has thrived and seen a massive increase in numbers and distribution.
What is extremely rare is that the colony is chlamydia-free.
Sydney koalas 'got lucky'
In some parts of Australia 90 per cent of koalas are infected with the sexually transmitted disease which can cause cystitis and its associated pain, blindness, infertility and even death.
"I think it's pure luck," now retired researcher Robert Close said.
"There's no clinical sign of chlamydia."
Dr Close studied the population for 24 years while working at the University of Western Sydney.
"The koalas weren't known in the area before 1986," he recalled.
The population started small but grew steadily and disease-free.
"Once the females established themselves they didn't die till old age and they gave birth to a cub at least once a year," he added.
"We saw grandbaby koalas in the population."
Dr Close believes the vegetation of city sandstone and Cumberland plain woodland soils and close proximity of the river have allowed the koalas to survive longer than other colonies.
"It gives them a great range of eucalypts to shelter on."
Threat of contamination
While the Campbelltown koalas remain STI free, there is a potential threat for cross-contamination with the colony in the Southern Highlands.
There is evidence of koalas, particularly males, travelling west and south to find new homes.
The Wollondilly Shire in partnership with the Office of Environment and Heritage has been investigating the potential for a crossover of the population.
"We're trying to ascertain whether the Hume Highway is a physical blocker that is holding the chlamydia back," Damion Stirling, environmental education officer for Wollondilly Council, said.
"The Southern Highlands population do have chlamydia and we're trying to work out if there is a crossover of the population.
"There's the potential but still an unknown factor."
Chlamydia doesn't necessarily mean the end of a population according to Mr Stirling, who said there were indications those in the Southern Highlands were still breeding.
Fears for habitat loss
What is more concerning for Mr Stirling and environment groups in Campbelltown though is the threat of habitat loss due to urban development.
Widening a three-kilometre section of Appin Road, which will "unlock new housing and aim to reduce travel times", has been proposed by the State Government.
"My concern is that these development proposals are working on a broadacre perspective and not looking at a more localised ecology," Mr Stirling said.
"Road issues create isolation of populations ... this leads to health issues and issues with inbreeding."
In the past few months 13 koalas have been killed on Appin Road.
Nine koalas have been hit in road strikes in the past two weeks further south, which Mr Stirling said was a result of koalas trying to cross the road to access red gum trees for food.
Koalas have also been seen in suburban street trees and on house roofs — an indication that the population was becoming more urbanised due to habitat loss and lack of food in natural bushland.
Mr Stirling said government departments needed to "open up communications" to better balance development with conservation.
"The things we need to see is the protection of corridors to protect habitats, restore buffer zones to restore food trees," he said.
"We need to ensure these developments have the appropriate fencing and protection mechanism so domestic animals like dogs and cats don't head into the area."
A Roads and Maritime Services spokesperson said if the development project proceeded, it would take "all necessary measures ... to avoid, minimise or offset the impact to the koala population".
One solution National Parks and Wildlife is pushing for is to give the bushland lining the Georges River from Glenfield to Appin national park status to protect it from development.
Dr Close recently handed over his database with two decades' worth of tracking information and research about the koalas of Campbelltown to the National Parks and Wildlife's Macarthur office.
Volunteer Pat Durman is preparing a proposal to maintain the green corridors between Campbelltown and bushland further south to avoid overcrowding.
"At the moment there's nothing really to stop development along the river," she said.
"Creating a new national park or conservation area will at least protect the corridor along the river.
"We need an environmental centre here based on koalas.
"We've got beautiful bushland, we've got one of the cleanest swimming holes in the basin, and we're lucky so we'd like to hang on to it — then we can share it with other people."