Objectives: To understand complaint risk among mental health practitioners compared with physical health practitioners. Design: Retrospective cohort study, using incidence rate ratios (IRRs) to analyse complaint risk and a multivariate regression model to identify predictors of complaints. Setting: National study using complaints data from health regulators in Australia. Participants: All psychiatrists and psychologists ('mental health practitioners') and all physicians, optometrists, physiotherapists, osteopaths and chiropractors ('physical health practitioners') registered to practice in Australia between 2011 and 2016. Outcome measures: Incidence rates, source and nature of complaints to regulators. Results: In total, 7903 complaints were lodged with regulators over the 6-year period. Most complaints were lodged by patients and their families. Mental health practitioners had a complaint rate that was more than twice that of physical health practitioners (complaints per 1000 practice years: psychiatrists 119.1 vs physicians 48.0, p<0.001; psychologists 21.9 vs other allied health 7.5, p<0.001). Their risk of complaints was especially high in relation to reports, records, confidentiality, interpersonal behaviour, sexual boundary breaches and the mental health of the practitioner. Among mental health practitioners, male practitioners (psychiatrists IRR: 1.61, 95% CI 1.39 to 1.85; psychologists IRR: 1.85, 95% CI 1.65 to 2.07) and older practitioners (≥65 years compared with 36-45 years: psychiatrists IRR 2.37, 95% CI 1.95 to 2.89; psychologists IRR 1.78, 95% CI 1.47 to 2.14) were at increased risk of complaints. Conclusions: Mental health practitioners were more likely to be the subject of complaints than physical health practitioners. Areas of increased risk are related to professional ethics, communication skills and the health of mental health practitioners themselves. Further research could usefully explore whether addressing these risk factors through training, professional development and practitioner health initiatives may reduce the risk of complaints about mental health practitioners.