Ultra-basic, strongly reducing conditions combined with depleted magnesium and sulfate proportions were observed in groundwater down-gradient of the Twin Hills gold mining operations in North Queensland. These conditions are atypical of groundwater systems beneath metalliferous mining sites, which are often characterised by acidic to circumneutral pH, minor to moderately oxidising redox conditions and elevated sulfate proportions. Beneath most areas of the Twin Hills mine site, evapotranspiration during recharge is the dominant mechanism controlling the solute concentrations in the groundwater. This has led to moderately saline (EC ~5,000 µS/cm), circumneutral pH (6-8) groundwater with major ion proportions similar to rainfall of the region. The sulfate-chloride ratios in most areas suggest limited alternative sources of sulfate (gypsum dissolution and/or sulfide oxidation). One monitoring bore situated down-gradient of the mining operations has historically exhibited significantly higher salinity (EC ~ 15,000 µS/cm), ultra-basic pH values (pH > 11), extremely low redox potentials (Eh < -200 mV), unique major ion signatures, and elevated concentrations of numerous metals/metalloids. Hydrochemical and mineralogical observations were combined with geochemical modelling techniques to assess the roles of various mine-related and/or natural mechanisms. The observed conditions likely evolved naturally from chemical weathering of olivine and pyroxene group minerals associated with olivine-gabbro intrusives. These mechanisms, collectively referred to as serpentinization, are solely responsible for the natural origin of ultra-basic pH and strongly reducing conditions that are rarely observed in groundwater systems, but are well documented in the literature. These conditions have led to sulfate and magnesium depletion due to sulfate reduction and precipitation of brucite (Mg(OH)2), which was observed to be present as significant mineral deposits at the base of the monitoring bore. This study highlights the importance of rigorous data collection and interpretation to identify causes and implications of groundwater quality “issues” beneath mine sites.