This paper presents two new efficient algorithms for estimating the capacity reliability of ageing water distribution systems recognising the uncertainties in nodal demands and the pipe capacity. Capacity reliability is defined as the probability that the nodal demand is met at or over the prescribed minimum pressure for a fixed network configuration. Uncertainties in the nodal demands and values of pipe roughness are modelled by a probabilistic approach. The impacts of these uncertainties on the hydraulic performance of water distribution systems are then assessed by probabilistic hydraulic models based on the mean value first order second moment (MVFOSM) method and the first order reliability method (FORM) respectively. The performances of the models are evaluated and compared by application to an example network. Results from this application indicate that both models provide reasonably accurate estimates of capacity reliability of a deteriorated distribution network in the cases that the uncertainty in the random variables is small. However, FORM performs much better in cases involving large variability in the nodal demands and pipe roughnesses.