Impairments to standing balance, gait and executive-cognitive functions are commonly observed in Parkinson's disease (PD) population. These problems are often exacerbated during the Dual-Task (DT) conditions that require simultaneous processing of executive-cognitive functions and sensorimotor resources. Lack of a reliable and valid instrument is a major limitation in DT assessment for PD population. A Computer Game Based Rehabilitation System (CGBRS) has been developed for simultaneously assessing balance, gait and executive cognitive functions in individuals with PD. The Main objectives of this study were to examine test-retest reliability and construct validity of CGBRS for evaluating the balance, gait, visuo-motor and visio-spatial executive cognitive functions under both single and DT conditions in individuals with PD. The present study also compared the effects of DT-interference on gait and executive-cognitive functions for stage-2 (mild) and stage-3 (moderate) individuals with PD. Twenty-six individuals with PD (stage-2 and 3, Hoehn and Yahr scale) were recruited for this study and examined on two separate occasions (one week apart). Moderate to high test-retest reliability was observed for performance measures of standing balance, gait, visuo-motor and visuospatial executive cognitive functions. A significant DT-effect was found for the majority of standing balance measures, spatio-temporal gait parameters, visuomotor and visio-spatial gaming task was observed in individuals with PD. Significant differences between stage-2 and stage-3 PPD were observed during single and DT-walking conditions. This study demonstrated the reproducibility and validity of the CGBRS for studying DT-interference and fall risk assessment in PD population.