Sixteen-year-old girls are incurably romantic"" comments Dr. Helwig, the imposing female anthropologist. And this, undoubtedly, has been the guide line of this author's many, many popular books for adolescent girls. There are all the necessary attractions here: an intriguing location--outback Australia (shown at its most exotic, with secluded ranch, aborigines, an old and later a new opal mine, colorful slang, and strains of ""Waltzing Matilda"" in the background); a mystery --bank robbery; and a third-year M.I.T. geology student as counterpoise to Carlie Copeland, making the most of her three week vacation from Chicago. As usual, the story is credible, the dialogue smooth.