Following an initial feasibility investigation and then detailed planning and design studies, a 650 km transmission interconnection in Western Australia came into service in the latter part of 1984. Since that time, there has been a rapid increase in load demand, and planning for the first reinforcement of the interconnection has now been completed. In view of the extended transmission distance involved, the crucial dependence on shunt compensation, and further loadgrowth projections, an extensive series of direct measurements has been undertaken for the purposes of examining as closely as possible the response of the system for a range of operating conditions. Measurements in the steady state include those of operating unbalance, voltage profiles, and harmonic distributions. Measurements related to transient modes include those of compensator energisation, shunt-capacitor switching, and controlled short-circuit faults. The paper seeks to report the main findings from these measurements and it includes selected test recordings from them. In one aspect, the tests make possible detailed assessments of the closeness with which computer-simulation methods can reproduce system operating conditions. To that end, comparisons are made in the paper between the results of live-system tests and those from computer simulation.