The Australian Koala Foundation said the number of koalas has decreased because of urban development and climate change, the Sydney Morning Herald reported Tuesday.
The group said the mainland population had fallen to between 43,000 and 80,000.
The figures suggest a drastic decline since the previous national estimate in 2003, which used similar research to arrive at a population of 100,000. Some scientists think the population is significantly larger, though most agree they are declining, the newspaper reported.
Twenty of the world's leading koala experts at a workshop in Canberra Wednesday are scheduled to determine whether the species should be listed as vulnerable, the Sydney newspaper reported.
"I think it will be very hard for anyone to walk away from this data. We have a very high level of confidence that it accurately shows a drastic decline," Deborah Tabart, the foundation's chief executive, said.
The Australian federal government has endorsed a national conservation strategy for koalas and will take under advisement the discoveries of the threatened species committee when they are available, Peter Garrett, a spokesman for the Environment Ministry, said.