Introduction Postnatal mental health problems, which are an international public health priority, are a suitable target for preventive approaches. The financial burden of these disorders is borne across sectors in society, including health, early childhood, education, justice and the workforce. This paper describes the planned economic evaluation of What Were We Thinking, a psychoeducational intervention for the prevention of postnatal mental health problems in first-time mothers. Methods and analysis The evaluation will be conducted alongside a cluster-randomised controlled trial of its clinical effectiveness. Cost-effectiveness and costs-utility analyses will be conducted, resulting in estimates of cost per percentage point reduction in combined 30-day prevalence of depression, anxiety and adjustment disorders and cost per quality-adjusted life year gained. Uncertainty surrounding these estimates will be addressed using non-parametric bootstrapping and represented using cost-effectiveness acceptability curves. Additional cost analyses relevant for implementation will also be conducted. Modelling will be employed to estimate longer term cost-effectiveness if the intervention is found to be clinically effective during the period of the trial. Ethics and dissemination Approval to conduct the study was granted by the Southern Health (now Monash Health) Human Research Ethics Committee (24 April 2013; 11388B). The study was registered with the Monash University Human Research Ethics Committee (30 April 2013; CF12/1022-2012000474). The Education and Policy Research Committee, Victorian Government Department of Education and Early Childhood Development approved the study (22 March 2012; 2012_001472). Use of the EuroQol was registered with the EuroQol Group; 16 August 2012.