New Zealand sea lion pups are born covered with thick, dark hair. It is unknown when they lose this natal coat. Adult males become dark blackish-brown with a well-developed mane reaching to their shoulders. Females have lighter coloration, generally buffy or grey with a lighter ventral side. They may have darker pigmentation around their flippers and muzzle. There is marked sexual dimorphism, also, in size of males and females. Males reach a maximum length of up to 350 cm, while females reach a maximum length of up to 200 cm. Males may weigh as much as 410 kg and females as much as 230 kg.

Dental formula is usually I 3/2, C 1/1, cheekteeth 6/5.

Range mass: 136 to 410 kg.

Average mass: 0.273 kg.