This paper develops a new methodology for allowing researchers outside central banks to test the extent of payment system risk, and applies this methodology to an investigation of the impact that the introduction of real time gross settlement (RTGS) had on systemic risk in the Australian payments system. System-specific ratios are first developed to extract bilateral payment obligations from aggregate payments data in the Australian RTGS system. This synthetic data is then used to generate a deferred net settlement (DNS) system of similar dimensions to those the Australian system would have had, had RTGS not been introduced. Standard default simulation methodology is then applied to test the levels of systemic risk in both this system and the corresponding RTGS system to ascertain the degree to which the introduction of RTGS is likely to have reduced the level of risk. We find that while the level of systemic risk is likely to have been reduced in the Australian case, the size of the effect is small, a finding consistent with the results of payments system studies in other countries.