This study evaluated the potential of monitored natural attenuation (MNA) as a remedial option for groundwater at a long-term petroleum hydrocarbon contaminated site in Australia. Source characterization revealed that total petroleum hydrocarbons (TPH) as the major contaminant of concern in the smear zone and groundwater. Multiple lines of evidence involving the geochemical parameters, microbiological analysis, data modelling and compound-specific stable carbon isotope analysis all demonstrated natural attenuation of hydrocarbons occurring in the groundwater via intrinsic biodegradation. Groundwater monitoring data by Mann-Kendall trend analysis using properly designed and installed groundwater monitoring wells shows the plume is stable and neither expanding nor shrinking. The reason for stable plume is due to the presence of both active source and natural attenuation on the edge of the plume. Assuming no retardation and no degradation the contaminated plume would have travelled a distance of 1,096 m (best case) to 11,496 m (worst case) in 30 years. However, the plume was extended only up to about 170 m from its source. The results of these investigations provide strong scientific evidence for natural attenuation of TPH in this contaminated aquifer. Therefore, MNA can be applied as a defensible management option for this site following significant reduction of TPH in the source zone.