Formal modelling is now widely applied for creating models of safety-critical interactive systems. Most approaches built so far either focus on the user interface or on the functional part of a safety-critical interactive system. This paper aims to apply formal methods for modelling and specifying the user interface, interaction and functional aspects of a safety-critical system in a single model using Coloured Petri Nets(CPN). We have used CPNs because of its expressive graphic representation and the ability to simulate the system behaviour. The technique is illustrated through a case study of the NikiT34 Infusion Pump.