The levels of voltage unbalance which exist in some interconnected sub-transmission systems in Australia have been observed to exceed the stipulated levels. The causes of the problem are not well known to system operators, i.e. whether the problem mainly arises as a result of system asymmetry or loads or both. Hence, system operators have found difficulties in identifying potential mitigation approaches which provide the optimum level of correction at various busbars. This paper reports, employing a study network, a suitable theoretical basis that can be used to identify the level of contribution made by the asymmetrical transmission lines and the unbalanced loads in relation to the problem of voltage unbalance and a systematic approach for identifying the optimum mitigation options.