Around long, Newton's parakeet was roughly the size of a rose-ringed parakeet. Its plumage was mostly greyish or slate blue in colour, which is unusual in "Psittacula", a genus containing mostly green species. The male had stronger colours than the female and possessed a reddish instead of black beak, but details of a mature male's appearance are uncertain; only one male specimen is known, and it is believed to be immature. Mature males might have possessed red patches on the wing like the related Alexandrine parakeet. Both sexes had a black collar running from the chin to the nape, but this was clearer in the male. The legs were grey and the iris yellow. Some 17th-century accounts indicate that some members of the species were green, which would suggest that both blue and green colour morphs occurred, but no definitive explanation exists for these reports. Little is known about its behaviour in life, but it may have fed on the nuts of the bois d'olive tree, along with leaves. It was very tame and was able to mimic speech.