Last week, an ecologist at the University of Sydney estimated that nearly half a billion animals had been wiped out since Australia's devastating wildfires started spreading in September. Now, he says that number has soared to over 800 million in New South Wales and over 1 billion nationally.

"I think there's nothing quite to compare with the devastation that's going on over such a large area so quickly," University of Sydney professor Chris Dickman told CBS News on Tuesday. "It's a monstrous event in terms of geography and the number of individual animals affected."

The figure includes mammals (excluding bats), birds and reptiles. It does not include frogs, insects or other invertebrates.

Get Breaking News Delivered to Your Inbox

"We know that Australian biodiversity has been going down over the last several decades, and it's probably fairly well known that Australia's got the world's highest rate of extinction for mammals," he continued. "It's events like this that may well hasten the extinction process for a range of other species. So, it's a very sad time."

Nearly 20 million acres have burned across the country, and authorities say the fires could keep burning for months. At least 25 people have died and residents of entire towns have been forced to flee their homes.

Dickman said that because Australia often sees the effects of climate change before other parts of the world, these fires could be a preview of what's to come globally.

The original estimate of 480 million animals killed int the fires was very conservative, Dickman said, and also included only the state of New South Wales. Since the fires have spread to other states, the number has escalated significantly.

The World Wildlife Fund Australia echoed Dickman's estimate. Based on his research, the wildlife conservation organization estimates around 1.25 billion animals have been killed directly or indirectly from the unprecedented fires.

"This heart-breaking loss includes thousands of precious koalas on the mid-north coast of NSW, along with other iconic species such as kangaroos, wallabies, gliders, potoroos, cockatoos and honeyeaters," the organization said on Tuesday.

The organization said that many of the country's forests will take decades to recover, and several species "may have tipped over the brink of extinction."

Dickman also stressed that not every animal included in his estimate will be killed directly by the fires. Some will go underground and re-emerge in areas that lack the proper resources to sustain them, and some will face new predators and be unable to compete.

The estimates are based on a 2007 report for WWF Australia. Researchers found that clearing around 1.6 million acres of forest would have resulted in the deaths of around 104 million native vertebrates through the loss of food and shelter.

Dickman came to his estimate after about 7.4 million acres of land had burned in the bushfires. That number has more than doubled in the last week.

Koalas, which were already under threat due to significant habitat loss, have been hit particularly hard. Ecologists estimated last week that nearly 8,000 koalas — about one-third of the population in their primary habitat — are believed to have died since the fires began.

While bushfires are common in Australia, this season's "mega-fires" are far from normal, WWF Australia said, with climate change and extreme high temperatures exacerbating the intensity of the fires.

Organizations continue to emphasize that the full extent of the damage will remain unknown until the fires subside.

"Due to the intensity, severity and size of the massive fires, few animals survive and the loss of wildlife and their habitat is heartbreaking," WIRES, an Australian wildlife rescue organization, said in a statement Tuesday.

The late Australian animal expert Steve Irwin's family announced that the wildlife hospital at the Australia Zoo, run by Irwin's widow, Terri, is busier than ever as it attempts to save animals rescued from the blaze. Irwin's daughter, Bindi, said they are "saving as many lives as we can."