There are parts of Australia where koalas are talked about like urban legends.
Older folk, who have lived in these areas for generations, tell stories about spotting the little Aussie icons in a favourite tree or hearing them bellow in the dead of the night.
Meanwhile, enthusiastic families can spend hours walking through that same bushland, peering at every nobby tree growth in the hope of it being a koala, only to end up with sore necks and complaining children.
Enter the acoustic recorder.
It’s been a game changer for our National Koala Monitoring Program, which has realised its latest estimates for this year. The estimates show an increase in the estimated numbers across Australia, which is good news for our koala friends.
Much of the change in the population estimate is driven by more survey effort, additional data, model improvement and more sensitive surveying techniques such as thermal drones and acoustic recorders.
These trusty little recording devices can tell us with high certainty whether koalas are present in an area or not.
With mating season underway in Queensland, our researchers have been hitting the road and have deployed about 150 acoustic recorders in Central and Northern Queensland. This will be followed by parts of South Australia throughout November and December.
These trips can result in researchers confirming koalas beyond their expected range. Last year they confirmed koalas at the edge of a range in Western Queensland. The male koalas were recorded bellowing near the small Queensland township of Quilpie, 478 km West of Roma.
Counting koalas
Koalas can be difficult to spot from the ground. They are most active at night and often rest high in trees during the day.
To accurately estimate the number of koalas in Australia, CSIRO’s National Koala Monitoring Program, in conjunction with partners, deploys a wide range of survey methods including walking surveys, drone-based thermal imaging, acoustic monitoring, historical surveys and citizen surveillance via Apps such as Koala Spotter.
CSIRO ecologist Dr John McEvoy said acoustic recorders were a really important tool in CSIRO’s toolbox to ensure accurate, site-appropriate data collection for long-term koala conservation.
“We’ve been using the acoustic recorders more or less from the start, but we’ve really ramped up our usage of them in the past year,” he said.
“A lot of the areas where we're studying are scarcely populated by koalas. We are pretty sure koalas are there, but looking for them on foot is going be a huge effort. So instead of doing that you can put out a smaller number of recorders and leave them recording for you know, two weeks or even more. And if a koala calls somewhere in those two weeks, you're going to get them.”
Statistically speaking, a two-week window is a little on the generous side and most koalas, if present, will be heard within 10 days.
And when they call, there is no mistaking them.
The trusty male koala might be small and cute, but it bellows like a rock star.
The koala mating call
Male koalas bellow to attract females as well as to intimidate other males and get them to leave the immediate area.
“The male bellowing is booming and travels quite far away,” Dr McEvoy said.
“It's good that it's so distinctive and so loud because that really does help us identify the koalas on the recorders.”
Male bellowing generally peaks around midnight, so recorders are usually set for taping from just after dusk until just before dawn.
This also avoids the dawn and dusk periods, where there’s a lot of general noise from birds, frogs, insects and other animals. The female koalas do vocalise, usually when the males are quite close to them, but it’s a much more high-pitched sound and harder to pick up on recorder.
How the recorders work
The omni-purpose recorders are not cheap (about $600 dollars) but their ability to gather information is enormous. A research team at the NSW Department of Primary Industries (DPI) developed a koala call recogniser and generously made the software freely available for other koala researchers to analyse their audio files.
The recorders are usually attached strapped to a medium sized tree, around shoulder height, and are quite small and camouflaged. They are programmed with a recording schedule to switch on and off and can be controlled with a phone app.
Next steps
The 2025 national koala estimate suggests more koalas than previously estimated, with between 729,000 and 918,000. While this is good news, this may reflect increased monitoring efforts (as mentioned above) rather than a substantial population increase. Koalas remain impacted by habitat loss, climate change, vehicle strikes and disease.
The monitoring program is funded by the Federal Government, and the final national koala population estimates for Australia will be due in 2026. It is anticipated that future work will also assess population trends.
Dr McEvoy said when it comes to estimating koala numbers, researchers are using multiple methods to get the best data.
“There’s locations that have historical records of koalas but no one has seen one in quite some time, that’s when we will use a recorder,” he said.
“If we confirm koalas are there, then we will come back and do a more detailed, more focused survey. You might put up the drone at that point and build in more methods as you go. It's rarely a case that we will only use one method and they all have their advantages.”
And the advantage of acoustic recorders?
“They’re a pretty basic piece of kit, but they’re pretty powerful,” Dr McEvoy said.