Monitoring the fluid chemistry at CO2 storage sites is essential for evaluating storage security and monitoring effectiveness. This study focused on the development and interpretation of benchmark geochemical datasets of formation fluids collected at CRC-3 injection well at the Otway International Test Centre (OITC) as part of the GeoCquest Field Validation project. The derived baseline datasets were used for the evaluating of pre-injection conditions, for post-injection fluid monitoring and the prediction of CO₂-induced changes in formation water composition. Importantly, the derived benchmark fluid data were compared with formation water data from an experiment conducted in 2014 to assess long-term geochemical variability. Baseline samples from the upper (1494–1499 m MD) and lower (1503–1513 m MD) zones at CRC-3 were classified as Na–Cl type waters, with HCO₃-alkalinity of 695–704 mg/L. Fluids were slightly alkaline (pH 8.1–8.5), enriched in K, Ca, Mg, and SO₄, and contained trace concentrations of Fe and Mn. Major ion concentrations (Na, K, Cl, SO₄) were significantly higher in baseline samples than in 2014, likely reflecting contamination from drill mud used during CRC-3 completion. The U-tube system in the upper zone was flushed frequently over several weeks with the aim to reduce this contamination. However, only a minor decline in the concentration of these major ions was observed. Minor but detectable CO₂ invasion was also detected in the upper interval, evidenced by lower pH, elevated alkalinity, and increased dissolved Fe concentrations. The potential source of increased Fe concentration are possibly wellbore materials and/or Fe-bearing minerals (e.g., siderite). These findings highlight the importance of distinguishing operational artifacts from true reservoir responses, ensuring reliable interpretation of geochemical monitoring data. The outcomes provide practical insights for improving monitoring strategies in future carbon storage projects and enhancing confidence in the environmental safety of geological CO₂ sequestration.