To reconstruct an indigenous language known solely from historical wordlists, the linguist needs to decide which source wordlists are most relevant, i.e. which sources are most likely to be attestations of the language to be reconstructed. There is little published research on methods appropriate to this task, and yet there is increasing attention to indigenous language reconstruction in support of language revival and revitalisation in education and community contexts. This paper describes a replicable and relatively objective method for comparing lexical similarity within a set of historical sources. The method described draws on the use of measures of lexical similarity in linguistics and the use of measures of endemism in biogeography. The method is illustrated via an analysis of historical sources for Aboriginal languages from the greater Sydney region, New South Wales, Australia. The sample is used to describe the overall similarity and difference between wordlists from this region, and to identify which wordlists are most similar to the wordlist recorded by the surveyor R. H. Mathews (e.g. 1903) as Darkinyung language.