Environment officials say the program is necessary to ensure the long-term welfare of the koala population and the preservation of natural bushland.

It's estimated there are about 150,000 koalas across the Mt Lofty Ranges with high densities in areas of Manna Gum woodland.

"In one area of woodland in the central hills, 13 koalas per hectare have been recorded and we are now seeing severe impacts due to over-browsing," Natural Resources Adelaide regional director Brenton Grear said.

"Optimal koala densities to prevent over-browsing of their habitat and ensure the long-term welfare of the koalas is around one per hectare."

Mr Grear said there was considerable evidence of over-browsing of preferred food trees, with severe defoliation, dead or dying trees.

"In effect, one of the greatest threats to the koala population in parts of the Mt Lofty Ranges is the koala population itself," he said.

The fertility program involves capturing individual animals to administer a hormone implant in a process that takes less than 10 minutes.

It is targeting areas of high koala over-browsing and follows the success of a long-standing program on Kangaroo Island that has led to reduced numbers there.

Australian Associated Press