Many migratory species are under threat globally. Management actions to conserve these species require a quantitative understanding of their life-history requirements, such as cues for migration. Migrations to spawn are a critical component of the life-cycle of diadromous fishes but are often poorly understood. Downstream migration patterns of common galaxias (Galaxias maculatus) were investigated in the Bunyip–Tarago River system, southern Australia, using passive integrated transponder technology. Fish undertook rapid (typically 1–3 days) downstream migration (up to 50 km) from the upper reaches to the lowland reaches near the estuary. Migration occurred from March to May during the known spawning period, with peak movement between late April and mid-May. Migration was also related to increased river discharge, with fish being more likely to migrate on days when flow was high compared with the previous 2 days, and a waxing moon (i.e. increasing illumination). Our results fill an important knowledge gap in the life history of this widely distributed diadromous species. Importantly, the knowledge about the likely drivers of movement can inform management actions such as providing environmental flows in austral autumn and at appropriate moon phases to trigger the downstream migration of adult common galaxias from freshwater reaches to estuarine spawning areas.