The kanamaluka/Tamar Estuary is a mesotidal, drowned river valley that at 71 km is Tasmania’s longest and largest, and also Australia’s longest navigable, estuary. The Tamar Estuary and Esk Rivers (TEER) Program has run a monthly survey (Ecosystem Health Assessment Program – EHAP) along the main axis of the waterway for fifteen years (2009–2024)1. The water column has been acidifying over the survey interval, seen as a monotonic decline in pH. It is most pronounced in the oligohaline (0.5–5) zone of the upper estuary, but is still distinguishable from marine waters up to a salinity of ~20. Using TEER-EHAP data coupled with physical and environmental information from the catchment, drivers of pH change have been evaluated. These have included climate change; increased inputs of organic matter (also indirectly via excess nutrients); acid drainage—proximal and remote; low-pH, point-source discharges (local industry/utilities or Trevallyn Dam); and acidic atmospheric deposition. The results of the investigation indicate that no single factor is dominant, but that a combination of factors is contributing to the acidification of the upper estuary.