An unverified component of Australia’s rich sea snake fauna is H. caerulescens, which is known from nine specimens collected in the Gulf of Carpentaria and provisionally identified by McDowell in 1972 based on their diagnostically high number of maxillary teeth. Micro-CT images of two paratypes of H. donaldi, which was thought to be endemic to Australia’s Gulf of Carpentaria, unexpectedly revealed a similarly high maxillary tooth count, prompting a re-assessment of both species. We examined 51 museum specimens and collated published morphological data for a further 55 individuals. Using counts of maxillary teeth, scale rows around the neck and body, temporal scale arrangement, and colour pattern, we identified all nine Australian records previously referred to H. caerulescens, and a single juvenile from southern New Guinea, were confidently identified as H. donaldi. This expands the known range of H. donaldi from a single locality in the Gulf of Carpentaria to include much of the fringe of the Gulf of Carpentaria, the Top End of the Northern Territory, and southern New Guinea. Hydrophis donaldi and H. caerulescens are not closest relatives; instead, their striking morphological similarity appears to represent yet another example of convergence in the taxonomically challenging Hydrophis. We present external morphological characters that distinguish H. donaldi from co-occurring congeners to support field identification of this poorly known species. Our study reduces the number of confirmed Australian sea snake species from 30 to 29 and underscores the need for targeted investigations of museum specimens to resolve remaining taxonomic uncertainties.