Ida Rentoul Outhwaite (1888-1960) was a prolific illustrator of children's books. She worked in watercolour but also in black and white. Her illustrations are delicate, intricate and, when not laden with too much sentiment, quite dramatic at times. What is wonderful though, flicking through a pile of loose prints such as those above, is that page after page is high quality b/w illustration of the kind one expects in such books and then suddenly, because she is an australian illustrating australian children's books, there are kangaroos, platypus, kukaburra, and koala, sometimes all in the same image.
As well as the usual biography sites:
there is also this rather interesting page from St Marks Anglican church in Fitzroy, Victoria showing four painted windows done by IRO.
A Google Image Search will lead you deep into her pretty world of fairies and kangaroos.