A koala fertility program is being implemented in parts of South Australia in a bid to control booming numbers, despite the marsupial now being listed as a vulnerable species in others parts of the country.

Key points: It is believe about 150,000 koalas live in the Adelaide Hills and Mt Lofty Ranges

It is believe about 150,000 koalas live in the Adelaide Hills and Mt Lofty Ranges A hormone implant will be administered to around 200 koalas in the area

A hormone implant will be administered to around 200 koalas in the area Experts say koalas are on track to become extinct by 2050 in NSW

Experts have previously warned that koalas are on track to become extinct by 2050 in New South Wales.

It has prompted the World Wildlife Fund of Australia to push for koalas to be listed as an endangered species on the east coast of Australia.

But there is seemingly no such issue in the Adelaide Hills and Mount Lofty Ranges, with experts revealing high numbers are having a destructive impact on native vegetation.

A recent survey has estimated there are about 150,000 koalas in the Mount Lofty Ranges and Adelaide Hills, and 50,000 on Kangaroo Island.

Showcasing the issue, an Adelaide Hills resident was left shocked when a late-night intruder turned out to be a koala rummaging through her house in February.

The high numbers have prompted the introduction of a fertility control program in a bid to preserve the population and the natural bushland.

It is estimated there are about 150,000 koalas in the Adelaide Hills and Mount Lofty Ranges. ( Supplied: WWF Australia/Doug Gimesy )

Natural Resources Adelaide and Mt Lofty Ranges regional director Brenton Grear said there were very high densities in some areas of the animal's preferred 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," he said.

He said the optimal number of koalas to prevent over-browsing was around one per hectare.

"Over-browsing deteriorates the health of the trees, which are their food trees, and those trees eventually die," he said.

"The outcome is eventually the death of the trees but also potentially some pretty catastrophic deaths of koalas through no food source.

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

How does the fertility program work?

Mr Grear said the fertility control program had started this week and aimed to curb breeding to reduce the koala population without harming the animals.

Mr Grear said the process would only take about 10 minutes per koala. ( Brett Jarrett )

He said trained staff would capture the koalas one at a time, bring them to the base of the tree and administer a hormone implant, then release them.

"The whole treatment takes less than 10 minutes and has been developed to minimise disturbance to the animal," he said.

He said the result would be that koalas with the hormone implant would be infertile.

"The evidence that we've had from the use of this implant is that they are effective for either the life of the koala or if the implant is removed, and that can be upwards of 15 years," he said.

"But if it was required and things changed in the future, the evidence is that the implant can be removed and the animals becomes fertile again."

The program follows the success of a long-standing program on Kangaroo Island which has been in operation for more than 20 years.

It is hoped the program will treat up to 200 koalas in the area during May and June.