Millions of animals are dead and hundreds of thousands more will perish over coming days as a result of killer bushfires terrorising southeast Australia.

Native wildlife and agricultural livestock are among the fatalities, with already-endangered species at greater risk of extinction.

The extent of the carnage may never be known.

"The fires will have killed millions of animals ... mammals, birds, reptiles," Wildlife Victoria boss Megan Davidson said.

A number of Steve Shipton's cows lay dead after being killed in his paddock during a bushfire in Coolagolite, NSW. Picture: Sean Davey/AAP

And the threat is not over, with wildlife rescue groups likely to be helpless in many instances.

"It is largely a job of euthanasing at this stage, both livestock and wildlife," Dr Davidson said.

"They are so severely burned that there is nothing better you can do than end their suffering." In coming days more animals will die from starvation and heat stress as they battle to find food and shelter in their decimated habitat.

The nationally-endangered eastern bristlebird is one species of concern amid the fires which are straddling the NSW-Victorian border.

"There are only three populations in Australia and two are under threat," The University of Melbourne's Alan York said.

A juvenile kangaroo was caught in a fence trying too escape. Picture: Brad Fleet

One of the small birds' habitats is near Jervis Bay on the NSW south coast and the other is Mallacoota in Victoria's East Gippsland, where fires forced people on to the beach to escape flames.

"It is a bird with very limited flight capabilities so it's very difficult for it to get out of the way of fire," Prof York said.

The eastern bristlebird's other habitat on the Gold Coast has also faced struggles in recent times but is not currently under threat from fire.

Despite the scale of destruction caused by the fires, both Prof York and Dr Davidson were hopeful that animal populations could recover.

"People get worried that species will become extinct but it's more about population size and they will return," he said.

"It is sometimes surprising how quickly things will recover - as soon as conditions are good again they can very rapidly breed up."

Professor Davidson said everyone could help animals that were not only suffering from fires but from years of drought.

"It's grim, but we don't want people to despair."

A young girl looks at the burnt body of dead kangaroo while walking her dog along a scorched property at Mallacoota. Picture: David Caird

HOW TO HELP THIRSTY WILDLIFE

* Distribute containers of water outdoors, being sure to throw in some sticks and leaves so that insects have something to cling to

* Swimming pool owners should add "climb-out points" so animals don't drown

* Fruit tree owners should remove netting to share their produce

* When offering pellets and hay, be sure to spread the food out so that species have a lesser chance of being targeted by predators