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.
