The study by Western Sydney University's Dr Edward Narayan examined 12,543 records of wild koalas at rescue sites in Port Stephens, Port Macquarie and Lismore between 1989 to 2018 and found disease was the most common reason they were admitted to care.

"The long-term trends for these koala hotspots paint a picture of a steady decline in populations," Dr Narayan said in a statement.

"(But), it's promising to see the majority of rescues - often undertaken by community groups and volunteers - have overwhelmingly resulted in the successful rehabilitation and release of koalas back into the environment."

Dr Narayan said the analysis indicated that as disease increased in local populations, successful rehabilitation and release decreased.

"This trend highlights the growing pressure care and rescue groups and their resources are under," he said.

The study found the age of a koala was a significant factor when it came to prognosis and recovery across all sites, but sex was not.

Dr Narayan said protecting the koalas' environment was the best way to support the work of rescue groups, stabilise the populations and reverse the trends identified.

"There is an urgent need to strengthen on-ground management, bushfire control regimes, environmental planning and governmental policy to reduce stressors impacting koalas on the North Coast and across the state."

The federal environment minister earlier this week approved a controversial quarry expansion in the Hunter that opponents say will destroy koala habitat, with the minister saying only "one or two" of the animals were found on the site.

Fifty-two hectares will be cleared to make way for the Brandy Hill Quarry extension in Port Stephens, after Environment Minister Sussan Ley twice delayed her decision as calls to reject the proposal grew.

Ms Ley said she had approved the project because quarry company Hanson had promised to establish a 74-hectare koala habitat corridor, 22 hectares more than the venture would destroy. "I recognise that the proposal has been subject to a high profile public campaign that has tapped into the genuine concerns we all share about koalas and bushfire impacted areas," Ms Ley said.

Hunter-based wildlife conservation scientist Ryan Witt released a report last month and told the Australian Community Media there was a "likelihood of healthy males and females utilising that habitat". But the author of the report used by the federal government to justify approving the expansion, Stephen Phillips, said "robust technique" used in the survey was "best practice".