A koala colony in Australia has been devastated by savage bush fires sweeping across a coastal reserve in the southeastern state of New South Wales.

Experts said that around 350 koalas in the Lake Innes Nature Reserve had been wiped out by the fires.

Koala Conservation Australia President Sue Ashton said there was a total population of 500 to 600 koalas in the reserve prior to the blaze.

The New South Wales Rural Fire service issued warnings for 17 fires, advising people to leave their homes in the state. More than 50 smaller blazes are also out of control, it said.

"These fires are very dangerous. If you are near these fires, your life is at risk and you need to take action to protect your life," NSW Rural Fire service said.

Rescued koalas are being treated at the Port Macquarie Koala Hospital, where animal carers have been feeding them eucalyptus leaves and formula and bandaging their wounds.

"We look for signals of pain - teeth grinding, distress - and we just take it on a day-by-day basis," said Amanda Gordon, who leads the team of carers, adding that some of the marsupials' health problems can be hard to spot.

Population estimates for koalas, native to Australia, vary widely, from as few as 50,000 to little more than 100,000.

Warmer weather brought by climate change has threatened the koala's natural habitat, said James Tremain of the Nature Conservation Council of New South Wales.

"Devastating bush fires are going to knock out some of these key population centres, but so also will increasing temperatures," he said, by affecting the nutrition value of the leaves that are the animals' sole food source.

"Koalas are definitely in trouble in New South Wales, but if the declines continue at the same rate as the last 20 to 30 years, koalas could be extinct in the wild by mid-century," Tremain added.

More than 1,000 firefighters and 70 aircraft had been deployed to battle the blaze, fire authorities told the ABC.

"It is a very volatile and very dangerous set of circumstances that we are experiencing right across these fire grounds in New South Wales," New South Wales Rural Fire Service Commissioner Shane Fitzsimmons said.

Australia has experienced a fierce start to its bush fire season, with an elderly couple killed in a blaze in October.

This is one of Australia's worst bush fire seasons, with a record number of emergency warnings and firefighters battling dozens of fires.