Screenshot of a video showing an Australian woman Toni Doherty rescuing a koala from a bushfire in New South Wales on November 19, 2019.

Exhausted firefighters said they had finally brought Australia's largest "megablaze" under control on Monday, as wet weather promised to deliver much-needed respite for countryside ravaged by bushfires.

New South Wales firefighters said they finally had the upper hand in the fight against the vast Gospers Mountain fire on Sydney's northwestern outskirts, which has been burning for almost three months.

Visiting the area on Monday, New South Wales Rural Fire Service Commissioner Shane Fitzsimmons said there was a "small area of burning still to complete" but the "containment prognosis looks promising."

The fire seared an area of national park three times the size of Greater London and lit several connected blazes totalling over 800,000 hectares.

As residents and authorities continued to come to grips with the sheer scale of the devastation, the Bureau of Meteorology forecast some firegrounds areas could get up to 50 millimeters of rain in the next week, a relief after a prolonged drought.

If that forecast bears out, the New South Wales Rural Fire Service said it would be "all of our Christmas, birthday, engagement, anniversary, wedding and graduation presents rolled into one. Fingers crossed."

Dozens of other fires are yet to be controlled.

The climate-change-fuelled fires have prompted an international outpouring and donations from around the world to help communities and animal populations.

Australia's unique flora and fauna has taken a catastrophic hit, with an estimated 1 billion animals killed, and countless trees and shrubs burned away.

The country's environment minister Sussan Ley has warned that in some areas, koalas may have to be reclassified as endangered.

The government has earmarked an initial A$50 million ($35 million) to spend on helping with the wildlife recovery.