12:20 (AEST) TOURIST demand for didgeridoos is taking a worrying toll on Top End bushland, it was claimed today.

Northern Territory environment authorities are planning a tagging system to curb the problem.

Estimates of the numbers of didgeridoos being produced reach the hundreds of thousands.

But few are covered by legally required NT Parks and Wildlife Commission (PWC) permits, said Josh Fornea, a PhD student researching sustainable harvesting.

"To give you some idea, I spoke to a few of the didgeridoo shops in Darwin and one of them told me they sold 9,000 last year alone," Mr Fornea said.

"Parks and Wildlife issued permits to make 3,000 didgeridoos across all of the Northern Territory."

The trade had taken a marked toll on the trees in Jawoyn land surrounding Katherine, 300km south of Darwin, where Mr Fornea has studied for the past few years.

No permits have been issued for the area.

"If you drive off the Stuart Highway a little way, you see literally hundreds of trees with missing branches or just cut down completely," Mr Fornea said.

The most frequently targeted tree in the area was the eucalyptus phoenicea - a less common eucalypt which is becoming increasingly scarce due to the illegal trade, he said.

"It seems to be a worsening problem," Mr Fornea added.

"It hit a crescendo with the 2000 Olympics when everyone was scrambling for as many didgs as they could get, but the trade is still going and getting bigger."

Aborigines were traditionally selective in their search for a tree that had been sufficiently hollowed by white ants before harvesting, he said.

But modern instrument makers - both black and white - were less selective and used power tools to hollow out unsuitable wood, he argued.

PWC joined Mr Fornea's research in the past year and is now considering developing a resource management plan that would include a trial tagging system for didgeridoos.

"There is no way for us to estimate how many more stems (didgeridoos) have been taken beyond the 3,000 permitted," PWC scientist Helen Puckey said.

"The tagging system would help us determine how many and where they're coming from."

Mr Fornea said the next step could be a tag of authenticity.

"Hopefully authenticity tags will be introduced to indicate a didgeridoo not only meets Parks and Wildlife sustainability criteria but it was also harvested or painted by Aboriginal people," he said.