Scientists have used bush recordings and technology similar to apps that identify music to show the rate of koala occupancy in the New South Wales hinterland is up to 10 times higher than previously thought.

In one of the most comprehensive koala population surveys ever undertaken, recording stations were set up across 1.7 million hectares of forest in north-east NSW, in an area stretching from the Hunter River near Newcastle up to the border of Queensland.

The idea was to capture the distinctive bellow of the male koala, which can be heard during the mating season in the summer months.

"We found koalas on average in 65 per cent of the sites we surveyed and that was surprisingly high. We really didn't expect that," Department of Primary Industries research scientist Brad Law said.

Dr Law said the ground-breaking technique showed there were a lot more hidden koalas in the bush than people thought.

Scientist Brad Webb uses a recording device to capture seven consecutive nights of sound. ( Supplied: Department of Primary Industries )

"Koalas are really difficult to find in some of the taller forests up in the ranges," he said.

"Whether it's looking for them with a spotlight or their scats in amongst the vegetation, it's not an easy thing to do.

"But during the breeding season the males have a loud call and at night this echoes throughout the forest, so by putting a sound recorder out there, it's proven to be a really great method, and the results we've got are good news for that north-east region."

Algorithm picks out koala calls

The survey suggests there could be 10 times the koala occupancy rate in north-east NSW than previously thought. ( ABC Open contributor Peter Crinion )

The project has been running for three years and overall more than 14,500 hours of bush noise was recorded.

The recordings were sent to an eco-acoustics lab at the Queensland University of Technology, where computer algorithms, similar to the music-identifying smart phone app Shazam, sifted out the koala calls from the ambient bush noise.

It is the first time eco-acoustics has been used to survey koalas, a process Dr Law described as a new and growing method of animal research.

"What we have now is quite a rigorous baseline of where the koalas are and what their occupancy levels are in those forests," he said.

"This new method gives us a relatively new and easy way to go out to capture that again, so we're hoping there will be ongoing annual monitoring across those forests so we can track how the koala populations are travelling over time.

"It just hasn't really been possible to do that in the past."

Coastal strip where koalas are under threat not included

Dr Law said the study did not give an estimate of the actual number of koalas, but an occupancy rate.

He said the study had not included the coastal strip, where it was well-known that koala populations were under threat from the pressures of urbanisation.

Koalas and humans are tiny blips in the forests of the Great Dividing Range. ( ABC Science: Ann Jones )

"I'd like to emphasise that without a doubt koalas in these areas are declining, but our research is suggesting that the hinterland is a stronghold," Dr Law said.

"This is a fairly big chunk of country where more than likely koalas are occurring at a low density, but they're very widespread in a lot of those forests."