Koalas are in rapid decline across the country, particularly in the regions where populations have the most genetic diversity, scientists say.

Koala numbers in NSW are on track to drop a third, and almost half in Queensland, over the next 20 years, according to Rebecca Johnson, chief scientist of the Australian Museum.

Koalas in bushland in St Helens Park, where numbers have increased over the past decade in a region where new housing developments threaten the local marsupial population. Credit:Nick Moir

South Australia and Victoria had more stable populations of the marsupials, but many are descendants of small numbers of transplanted animals after previous culling wiped them out.

In South Australia's case, the koalas - estimated at 33,000 in a 2016 paper compiled by Christine Adams-Hosking and others - descended from just six individuals, Dr Johnson, a conservation geneticist, said.