Heavy rains in fire-ravaged eastern Australia have brought welcome relief for firefighters and farmers, but sparked flash floods that have led to fresh scrambles to save native animals.

As the rain hit on Thursday the New South Wales State Emergency Services department warned that the sudden heavy downpours in some areas would bring flash flooding, falling trees and landslides where the fires have wiped out vegetation.

On Friday, the warnings were realised when flash floods hit the Australia Reptile Park on the NSW east coast, and the state's koalas - having lost thousands of their number and huge swathes of their habitat - needed to be rescued again as floods thundered down fire-blasted hills empty of vegetation.

Park director Tim Faulkner told local media that the sudden floods on Friday morning were “incredible”.

“Just last week we were having daily meetings to discuss the imminent threat of bushfires,” he said.

“Today, we've had the whole team out there, drenched, acting fast to secure the safety of our animals and defend the park from the onslaught of water… We haven't seen flooding like this at the park for over 15 years.”

And while the rains have doused fires in some areas, blazes continue to rage across many other parts of the country where the weather stayed dry, including in other parts of New South Wales where 82 fires were still burning, with 30 out of control, and in the state of Victoria, to the south. Parts of the state’s Alpine region were evacuated again as erratic winds caused spot fires around a large blaze at Mount Buffalo.