Geodorcus novaezealandiae

Including their mandibles, the male is 12–21.5 mm in length whereas the female is 11–17.4 mm, which makes them the smallest species of Geodorcus. Like all other Geodorcus, they show sexual dimorphism: the male has a much wider head and larger mandibles whereas the female mandibles are relatively short and are less apparent. In the male, the front of the pronotum (the upper part of the thorax) bulges forwards and slightly hangs over the head whereas in female the pronotum is normal. The body is dull to slightly glossy black, with the upper surface being covered in small punctures. The elytra (hardened plates that cover the abdomen) have a series of slightly raised ribs running down their surface towards the tip of the elytra. The frons and vertex, which make up the front parts of the head, have a distinct depression.

G. novaezealandiae is only found in New Zealand, where it is found in the south of the North Island. Its range extends from the Akatarawa Valley in the Tararua Range in the north of the region to the Aorangi Range in the south. It has been observed at altitudes from near sea level to 365 m. It has been observed under the bark of live rimu (Dacrydium cupressinum) and southern beech (Nothofagus) trees. It's also been found in old beech stumps and amongst roots. Like other Geodorcus species, it is forest-dwelling and nocturnal. Specimens have also been collected in traps 1.5 m above the ground on māhoe (Melicytus ramiflorus) and hinau (Elaeocarpus dentatus) trees. In one study, pitfall traps were able to collect adults in winter, spring and summer whereas a revision of the species had specimens collected from July to April.

As adults, it is presumed to feed on plant sap from the trees they hide in. It has been reported feeding on speargrass (Aciphylla) and on pieces of apple (when held in captivity). Like other Geodorcus species, the larvae feed on rotting wood. It has been speculated that both sexes work together to create tunnels in rotting wood to lay eggs in, with the male's large mandibles being well suited for the task. While copulating, the male and female face opposite directions with the tips of their abdomen touching.