Of course, the appearance of the nēnē-nui in life is unknown, but it can be assumed to have been similar to its extant relative, as the Branta geese share most plumage characteristics.

The chief differences to the nēnē were that it was usually about 5% larger, while the bones of the flight apparatus were reduced in length by about the same amount.

At the time of its extinction, the nēnē-nui was in an intermediary stage of the evolution towards flightlessness: the extensive material suggests that, in the same population, the entire span of individuals with diminished or marginal flight capacity to flightless individuals with markedly reduced wings had been present.

Thus far, the nēnē-nui is the only species in which the process of becoming flightless is documented by actual paleontological evidence, rather than just the end result.

A single skeleton of a similar bird has been found on Kauaʻi, and many more on Oʻahu; the latter seem to be intermediate between the nēnē-nui and the nēnē.