Atrial fibrillation (AF) is the most common arrhythmia. The risk of AF increases in postmenopausal women, suggesting beneficial effects of oestrogen. This study examines the effects of oestrogen at different concentrations corresponding to physiological states, including the menstrual cycle, pregnancy and menopause. We incorporate a hormone effect function, derived from experimental data, into an existing biophysical model of the atrial cardiomyocyte to simulate the effects of oestrogen. Our modelling results suggest that oestrogen may exert cardioprotective effects at higher physiological concentrations, such as those observed in the phases of the menstrual cycle and pregnancy. Specifically, it prolongs the action potential duration and facilitates the termination of isthmus-dependent re-entry in an AF substrate. These findings support the protective effect of oestrogen and provide insights into the role of oestrogen in AF dynamics, with implications for future studies to understand sex-specific arrhythmia risk and potential therapeutic strategies.