James M Downie was an Australian author. He wrote children’s stories, mostly based on flying, and flying doctors. Some of his books, including Warrigal, the Story of a Wild Horse, Killer-Dog, and The Treasure of the Never-Never, were illustrated by his brother, John C Downie.

Finding the books
Reasonably easy to find, though not necessarily with dustjackets.

Links and sources
Austlit - the Australian Literature Resource