Over a billion animals have been killed or displaced in bushfires throughout NSW this season, and "hundreds of billions" of insects have also died, according to leading wildlife experts.

Key points: Some species in NSW are likely extinct before being discovered and documented

Some species in NSW are likely extinct before being discovered and documented Cats and foxes pose a significant threat because trees — a typical refuge — have been destroyed

Cats and foxes pose a significant threat because trees — a typical refuge — have been destroyed Bushfires have now razed more than 5 million hectares in NSW

The figure has more than doubled from an original estimate of 480 million animals lost, as the hectares razed by out-of-control fires increased from 3 million to now almost 5 million in NSW.

Ecologist Chris Dickman from the University of Sydney said: "for some species we're looking at imminent extinction".

"There will almost certainly be species of all geographical ranges and populations that are cooked before we've even had the chance to discover that they exist," Professor Dickman said.

Wildlife is threatened by more than just flames in a bushfire crisis, says David Lindenmayer, a professor of forest ecology and management at Australian National University.

"Australian wildlife has to deal with four things: the incredibly fragile overheated periods before fires, the fire itself, the lack of habitat and food after the fire, and the fourth thing is the invasion of foxes and cats in these burnt areas," he said.

Bushfires critically endangered the greater glider whose thick pelt is prone to overheating. ( Supplied: ACF )

Professor Lindenmayer said there has never been a better time to keep domestic cats inside overnight.

"It's easy pickings for them — cats can move up to 12 kilometres through areas that have just been burnt," he said.

Professor Dickman said the aftermath may mean "species that are rendered extinct, ecosystems that have been eroded to the point where they are completely changed, and habitat in a state of widespread impoverishment".

"The loss of life we've estimated for NSW is 800 million terrestrial animals, including birds and reptiles. But that figure doesn't include frogs, fish, bats and invertebrates," he said.

"Combining these figures [it] is likely well over a billion animals lost."

The long-nosed potoroo is a secretive animal that likes thick vegetation. ( Kristian Golding: Flickr.com )

Professor Dickman said invertebrates — which include butterflies, spiders and earthworms — hold a critical place in the ecosystem, providing pollination, seed dispersal, soil health, nutrient recycling and an essential food source for a large number of marsupials, birds and over 90 per cent of all lizards.

"It's impossible to estimate the loss of life of invertebrates, but it'll be undoubtedly in the hundreds of billions," Professor Dickman said.

Editor's note July 29, 2020: An earlier version of this story quoted researchers as saying that 1 billion animals were killed in Australia's bushfires. The same researchers now say they meant to say that 1 billion animals had been "killed or displaced". Since then they have revised that figure again, to 3 billion.