Books in the Fantastical New Zealand series

Eight-year-old adventure seeker Pete is excited to explore a mysterious nighttime noise. His ten-year-old, more sensible, sister Trudy, thinks it sounds much too dangerous. But when it turns out to be a friendly pegasus in need of the brave sibling duo, can they put aside their differences to beat a vain mountain creature, or will the beauty of the Southern Alps disappear for good?