The new study, conducted by a team of researchers and scientists, followed an interim report by WWF in July that first estimated the toll of impacted animals to be three billion.
"It's hard to think of an event in the world, in living memory, that has impacted so much on wildlife and nature," chief executive of WWF Australia Dermot O’Gorman said.
"These numbers are really off the charts and they drive home the unprecedented nature of the Australian bushfires which really require a massive response."
More than 41,000 koalas on South Australia's Kangaroo Island alone are estimated to have been impacted by the fires, with another 11,000 in Victoria and 8,000 in New South Wales believed to have been caught up in the disaster.
Mr O'Gorman said this was a "devasting number for a species that was already sliding towards extinction". "We cannot afford to lose koalas on our watch," he wrote in a foreword to the report.
A NSW parliamentary inquiry in June found koalas would become extinct in the state before 2050 without urgent government intervention to stem habitat loss.
Conservation groups have also called on the government to designate the iconic Australian marsupial, which are only native to NSW, Queensland, Victoria, and South Australia, an endangered species in the wake of the bushfire crisis.
In addition to koalas, the report estimates nearly 40 million possums and gliders, 36 million antechinuses, dunnarts, and other insectivorous marsupials, 5.5 million bettongs, bandicoots, quokkas, and potoroos, five million kangaroos and wallabies, 1.1 million wombats and 114,000 echidnas suffered due to the crisis.
Researchers said a lack of data on wildlife populations in 11.46 million hectares of fire-affected land meant it was difficult to determine how many animals were killed by the blazes, as opposed to injured or forced to flee their habitat.
"Even if resident animals were not killed outright by fires and managed to escape, they will surely have experienced higher subsequent risk of death as a result of injuries or later stress and deprivation of key resources," the report read.
The report made 11 recommendations to ensure the recovery of wildlife populations after the crisis and better understand the scope of the crisis, including better mapping and monitoring of plants and animals in at-risk areas.
"More research is needed on how many animals are out there and their ability to survive different levels of fire intensity," said Lily Van Eeden from the University of Sydney, who managed the research. "We need to understand this to protect species more effectively."
Environment Minister Sussan Ley announced a $2 million national "koala audit" in November to rectify "a serious lack of data" about where populations actually are and how they have recovered following the fires.
Other recommendations included developing a national standard for monitoring wildlife populations, protecting and improving habitats, and establishing fire prevention and management strategies, which includes drawing on traditional knowledge and practices.
"People have been shocked by our research and have said to me ‘we can’t allow catastrophes of this magnitude to continue into the future’," said the University of Sydney's Professor Chris Dickman, who oversaw the research.
“With long-term monitoring, we would be in a much better position to know where and when to act and what resources are needed to save at-risk species."