A newly prepared elasmosaurid plesiosaur specimen, from the Haumurian (Upper Cretaceous) Conway Formation of North Canterbury, is more complete than any found previously in New Zealand. The specimen, a young adult, comprises posterior fragments of the skull, almost the entire vertebral column, fragments of the major bones of both pectoral and pelvic girdles, and portions of all four limbs. Comparison with the partial pelvic girdle and hind paddle that constitute the lectotype of Mauisaurus haasti Hector, 1874 (including the femur, diagnostic of the species), indicates that the new specimen belongs to that species, permitting a more complete description and diagnosis. The species is characterized by an estimated 86 presacral vertebrae, at least 66 of which are cervicals, other than the atlas and axis. The coracoids possess a pronounced, blunt, ventral process, and a broad, rounded, transverse ridge on their dorsal surface. In large adults the medial profile of the coracoid becomes markedly sigmoidal. The adult femur has a large hemispherical capitulum; the humerus is relatively broad with an elliptical capitulum. In life, the animal would have been in excess of 8 m long. Comparisons with previously described elasmosaurs are difficult because of barely adequate published descriptions, but Mauisaurus haasti is distinguished particularly by postcranial characters that vary among Late Cretaceous elasmosaurs from both the Northern and Southern hemispheres. The only other established New Zealand elasmosaur, Tuarangisaurus keyesi Wiffen and Moiseley, 1986, cannot be directly compared with Mauisaurus haasti and must remain a separate taxon.