Using a systems view of a work process, a task consists of a number of transformative interactions, performed simultaneously or successively, to a given level of effectiveness, and taking place at the interface between the operator and key components of the system; these encompass not only traditional hardware, software and workstations, but social, managerial, infrastructure and peripheral factors. We propose the Hexagon - Spindle Model for Ergonomics, which represents the multiple factors influencing the effectiveness of interactions at the interface. The model is holistic, multi-dimensional, task-related and transferable across a range of settings. It extends to characterise a time base for serial and simultaneous tasks and highlights areas where operator/system conflicts may arise. We illustrate analysis tools for the application of the model in evaluation and design.