ABSTRACT: Parasitic and non-parasitic lamprey ‘species pairs’ are an ongoing source of taxonomic uncertainty globally. The geographic range of the only non-parasitic lamprey in the Southern Hemisphere, the Endangered Australian brook lamprey Mordacia praecox, has remained ambiguous since its description in 1968. The conservation of this species is complex as it is presently genetically and, for most of its lifecycle, morphologically indistinguishable from its parasitic paired species, short-headed lamprey M. mordax, the conservation status of which is Least Concern. Difficulty in their identification, coupled with their cryptic behaviour, has resulted in limited knowledge of the species’ ecology and distribution. This is further complicated by the sympatric geographic ranges of the paired species. Using incidental captures, targeted surveys, and open-access wildlife information database records, we describe the discovery and confirmation of lamprey populations from tropical and sub-tropical Australia and the associated ∼1400 km (5-fold) northward extension of the known geographic range of Australian brook lamprey. Surveys yielded non-parasitic lampreys of all life-history stages across 6 tropical and sub-tropical coastal watersheds of eastern Australia. We also highlight major issues associated with the conservation of a cryptic and poorly understood species and discuss potential conservation actions that may, at least in part, ameliorate such issues.