The levels of voltage unbalance that exist in some interconnected sub-transmission networks in Australia has been observed to be above the code requirements (e.g. 1% in Victoria). There is a perception that these high levels arise as a result of asymmetrical loads. However, it is identified that the network asymmetry in relation to transmission lines also play a significant role in the lead up to this situation, employing an interconnected 66kV sub-transmission system as the study case. The individual transmission lines of the study network are analysed in order to observe their possible influence arising as a result of line impedance characteristics and line loading levels. A methodology is developed to quantify the contribution of individual transmission lines to the role played by the network at nodal level. The leading contributors to the problem in the study network are explored employing the proposed approach.