After wildlife carers were notified, the koala was rescued and transported to Werribee Open Range Zoo, where the animal underwent urgent medical treatment.

It's likely the koala slipped from a tree and fell into the river during the storms, before it washed up into the cave with no pathway out, senior veterinarian Dr Natalie Rourke said.

"The koala was quickly becoming weak from starvation because he had no access to his daily eucalyptus diet while trapped in the cave," she said.

"Koalas are not natural swimmers, so without intervention, it could have drowned attempting to exit the cave to find food."

The koala spent five days rehabilitating at the zoo before it was released back into the wild, near where it was found.