The level of voltage unbalance that exists 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, the system asymmetry (of lines which are not normally transposed at 66kV, transformers and capacitor banks) also can play a significant part in the lead up to this situation. This lack evidence is partly due to the fact that analysis of asymmetrical systems require careful modelling of system components and lack of widespread availability and use of proper unbalanced load flow programs. To understand the impact of transmission lines in an interconnected network in relation to voltage unbalance, individual and interconnected behaviour of transmission lines have to be closely investigated under balanced supply and loading conditions. The paper presents the methodology and the results of such an investigation carried out employing an interconnected 66kV sub-transmission system as the study case.