"It is the same type of contraceptive that is used by human females around the world," said project manager Ian Walker. "But the tricky part about this program is catching the koalas and getting them out of the trees."

The hormone is contained in a tube the size of a match stick and is inserted beneath the koala's skin, between its shoulders.

The furry marsupials can live for up to 18 years and may produce up to 11 offspring. A symbol of Australia, Koalas are protected but the eucalyptus trees they depend on for food are not.

"Victoria has the largest number of wild koalas of all states in Australia, so that means we need a plan that protects the species as well as the environment it depends on," said Victoria Environment Minister John Thwaites.

There are about 100,000 koalas in Australia, down from seven to 10 million at the time of European settlement in 1788. In the 1920s three million were shot for their fur.

Trials have shown the hormone implants, an alternative to relocation or surgical sterilisation, have prevented conception for six years.