Uncontrolled discharge of Acid Rock Drainage (ARD) from Mount Morgan mine site accumulated over time poses significant environmental risk on the Dee River downstream. This paper investigates and flow dynamics and the ARD dispersion and dilution along the Dee River for a number of dam-break scenarios, especially to estimate the extent of downstream reduction in concentration in extreme weather conditions. Hydrologic data of Dee River was analysed and a detailed 1-D hydraulic river model was set up using Danish Hydraulic Institute, Mike 11, to simulate the contaminant transport of ARD. Simulation results of different weather condition assessed to be used as resource to proposed management options to minimise the risk of uncontrolled discharge into natural waterways. Results indicate that if dam break occur in a dry to medium season there is a risk of contamination of the downstream catchment. But if dam fails in a medium to wet season downstream concentration will be diluted to minimum 1.8 PSU.