Shortfalls in our knowledge of the most basic parameters, such as overall range and population size, ensure evidence-based conservation of poorly known or ‘missing’ species is inherently difficult. Often, the only source of such knowledge is anecdotal reports, which are usually considered too unreliable to be of value. Methods that help conservation decision-makers use anecdotal records of poorly known or ‘missing’ species to decide where conservation action should occur, and how urgent that action might be, will support better conservation decisions for those species. Here, we use a Delphi-style process based on expert opinion to assess the largely anecdotal sightings record of the Night Parrot (Pezoporus occidentalis), an endangered species from arid central Australia that underwent a significant decline following the arrival of Europeans. Our results clarify the patterns and possible causes of this decline and subsequent range contraction. We conclude that the species persists in only two broad regions, and is probably extinct throughout much of its former range. Our method is applicable to other poorly known species with a similarly sporadic and largely anecdotal sightings record. This method could be used to clarify the historical and current distribution and status of such species, a critical first step in understanding their conservation requirements.