Australian kids book read in 50s, good vs evil battle with anthropomorphised Australian animals

[Sycorax]

My dad is after a book he read in childhood. He read it in the mid 50s, but the copy was battered enough that he thinks it was probably from an earlier decade. It was a novel with black and white line drawings, probably intended for kids maybe eight and older. It was missing a cover when he read it.

The plot involves a group of children - the main character was a boy - who stumble into and become embroiled in some kind of good vs evil battle. The good side are represented by benign Australian animals, like kangaroos, and the evil side are snakes, goannas etc. My dad says he found it quite sinister at the time. He couldn't remember anything about the title, author or names of characters.

Assistance greatly appreciated!