Forehead slopes steeply over a slightly bulging melon. The beak is short with a relatively straight mouthline.

Descriptions of coloration in M. hectori are lacking. The main accounts in the literature are based on specimens of that have subsequently been attributed to a separate species, M. perrini. Previous accounts describe an animal that is dark dorsally and pale ventrally.

Adult body length ranges between 4 to 5 m. Recorded maximum body length for adult males and females is 5 m and 4.3 m, respectively. Length at birth is 2 m.