These large, chunky birds are about 70 cm long and weigh about 2–3 kg, slightly larger than the closely related Foveaux shag.

A recent taxonomic revision argues that Leucocarbo is a distinct genus, which would contain amongst other species the Otago, Foveaux, and Chatham shags.

Archaeological evidence shows that Otago shags were formerly found along the entire east coast of the South Island up to Marlborough, but when humans arrived the population was devastated, reduced by 99 percent within 100 years with a corresponding loss of genetic diversity.

[8] Restricted to a small area, and having little or no genetic variation, they require conservation efforts tailored to these extinction risk factors; this could include reintroduction to part of their former range.

[4] Otago shags breed colonially from May to September, making raised cup nests out of organic material and guano on islands and sea cliffs.