What may be the newest addition to Otago's Hector's dolphin population was spotted by crew on the Monarch Wildlife Cruises and Tours boat on Tuesday.

Crew member Shaun Wilson saw the calf, which still had foetal folds (vertical creases along the side from being folded inside the womb), and mother, off Taiaroa Head.

University of Otago zoologist Professor Liz Slooten said about 40 Hector's dolphins were known to inhabit Otago waters.

Between 12,000 to 18,500 Hector's dolphins are thought to exist in South Island waters.