This study introduces a two-port network-based behavioural modelling approach for resonant-operated capacitive wireless power transfer (WPT) systems. A simple, generic and unified modelling approach is developed to describe the behaviour of WPT systems, under changes of the source and load circuits, variations of the coupling interface and drifts of the components in the matching networks. The resultant model provides insights into the electrical cross-coupling relationships between input and output parameters of the capacitive power transfer systems, including the effect of distance and alignment of the coupling plates. Regardless of the circuit complexity, it is demonstrated that the model core can be reduced to a basic gyrator relationship with added coefficients when required, thus obtaining a compact, closed-form relationship between the input and output. To provide a simulation framework for capacitive medium variations, a simulation-compatible model of the capacitive coupling using a continuous-time variable capacitor has been constructed. The behavioural model and methodology have been validated through simulations and experiments. A 200 W experimental capacitive WPT prototype has been designed and examined for various air-gaps up to 100 mm at a resonant operation of 1.56 MHz. A very good agreement is obtained between the theoretical predictions, simulations, and experimental results.