Newton's parakeet was about long – roughly the size of the rose-ringed parakeet. The wing of the male specimen was , the tail , the culmen , and the tarsus was . The wing of the female specimen was , the tail , the culmen , and the tarsus was . The male specimen was greyish blue (also described as "slatey blue") tinged with green, and darker above. The head was bluer, with a dark line running from the eye to the cere. It had a broad black collar running from the chin to the nape, where it became gradually narrower. The underside of the tail was greyish, the upper beak was dark reddish brown, and the mandible was black. The legs were grey and the iris yellow. The female was similar but had a greyer head and a black beak. The black collar was not so prominent as that of the male and did not extend to the back of the neck.