Tens of thousands of iconic Australian creatures including koalas and kangaroos may have died in fires that swept through vast tracts of southern Australia this week, environmentalists say.

The blazes have devastated thousands of hectares, razed clusters of homes and claimed one life since they began earlier this month.

But they will also leave a significant environmental legacy because of their impact on flora and fauna, according to Wildlife Victoria spokeswoman Sandy Fernee.

"I think we've already lost tens of thousands of animals when you consider how widespread the fires are," she told AFP.

The wildfires, caused by lightning strikes in some areas and arson in others, have raged in the southern states of Victoria and Tasmania as well as New South Wales and Western Australia for two weeks.

Victoria is the worst affected, with the scorched area stretching over more than 5,000 square kilometres (2,000 square miles), equivalent to twice the size of Luxembourg.

Fernee said volunteers visiting Victorian firegrounds this weekend were expecting to see "Pompeii-like" scenes of burned and charred animal remains.

"It's very grim. A lot of what we come across are animals we can't even recognise. It's just a pile of ashes," she said.

"It's rare that we find something we can help. Mostly we come across dead animals that are badly burned that we have to euthanise."

While koala, kangaroo and wallaby populations are expected to have been hit, smaller animals such as possums, bats, birds, lizards and snakes are thought to have perished in greater numbers because they are generally too slow to outrun a fire, she said.

Kevin O'Loughlin, head of a Melbourne-based national bushfire research centre, said assessing the specific damage could take years.

"But fires of this scale are disastrous environmentally," he told AFP.

"They really do cause a terrible lot of damage because of the area that they burn and the ferocity of the burn."

While some Australian plants were able to thrive on fire, with the flames opening seedpods, ecosystems would take years to recover from gigantic blazes which left nothing behind, he said.

Residents of fire ravaged areas could expect plants and wildlife to be incinerated, while water supplies could be tainted by ash, he said.

Air pollution has also been significant, with the southern city of Melbourne shrouded in such thick smoke over the past week that fire alarms have been detonated inside city buildings.

Stuart McConnell, director of science and technology at Victoria's Environment Protection Agency, said air pollution was five to 10 times worse than normal because of the fires.

"This is certainly the worst example of this (type of pollution) that we've seen," he said.

Hundreds of firefighters are battling to contain the blazes, but officials expect those in Victoria and Tasmania to continue to burn for weeks as they churn through rugged and largely inaccessible bush.

Geoff Law, a campaigner with the Wilderness Society in Tasmania, says the fires in the island state are contained to state forests and private land and are not threatening national parks.

"But there's still birds being incinerated and kangaroos and wallabies getting scorched and potentially dying of burns as well as all the smoke," he told AFP.

Ultimately, the extent of the damage would depend on the severity of the fires, he said.

"Where there's been a firestorm or fireball most of the trees will be killed. There will be significant patches where everything is dead.

"But there will be regeneration. But if the frequency is too great then you do get a change in vegetation -- from taller forest to something a bit more scrubby.

"We hope that for most of the areas that have been burned, that it hasn't been too intense."