Hundreds of koalas are feared to have burned to death in an out-of-control bushfire on Australia's east coast, wildlife authorities said Wednesday.

A bushfire believed to have been sparked by a lightning strike on Saturday some 248 miles north of Sydney has ravaged an area of over 2,000 hectares (4,942 acres) with authorities battling to bring it under control.

Wildlife rescuers in northern New South Wales state hold grave fears for a "very rare" population of hundreds of koalas living in the fire zone.

"The special importance of those koalas is that they are very genetically diverse," Sue Ashton, president of the Port Macquarie Koala Hospital, told AFP.

Her fears are that "hundreds" in the known koala breeding area "have perished in the fire".

"It's a national tragedy because this koala population is so unique," she added.