A z-transform signal model that when combined with a nonlinear post-filtering scheme is able to estimate the operating frequency and voltage/current phasors in a power system is developed. The signal model parameters are identified by an optimisation method in which the error between the model output and the actual signal that represents a voltage or current in the power system is minimised. The form and structure of the signal model do not require iterations in the optimisation process for parameter identification. The system operating frequency is directly evaluated from the model parameters. Noise effects and possible mismatches between the model and the actual signal are countered very effectively by applying a median post-filtering process to the time series representing the frequency estimates derived from the model. Extensive simulation studies and comparisons with existing frequency estimation techniques confirm the high performance of the proposed method in terms of accuracy and time delay. The accurate estimation of the operating frequency, achieved by the proposed method, allows voltage and/or current phasors in power systems to be measured or determined more precisely.