Issue addressed What Were We Thinking (WWWT) is a psychoeducation group program for the primary prevention of postnatal mental health problems in women. It addresses two neglected risks for postnatal depression and anxiety: unsettled infant behaviour and adjustments in the partner relationship after the birth of a first baby. WWWT has evidence of efficacy when facilitated by specialist nurses. It is designed to be facilitated by trained nurses as part of standard postnatal primary care. The aim of this study is to assess nurses understanding of their role in prevention of postnatal mental health problems, the requirements for and feasibility of implementing WWWT, and nurses current practice and specific training needs. Methods Maternal and Child Health (MCH) coordinators emailed nurses at all Victorian MCH centres an invitation to participate in a study about parent-infant mental health. Semi-structured interviews and group discussions were conducted. Responses were analysed thematically. Results MCH nurses demonstrated a willingness to implement a structured mental health-promotion intervention into clinical practice. Successful implementation will require changes to enable routine attendance of fathers at group sessions and recommendations to parents about the use of infant behaviour management strategies. Conclusions This study contributes to comprehensive knowledge exchange about the implementation of a complex intervention in standard primary care. This knowledge is essential for generating practice-based evidence of the WWWT mental health-promotion program s effectiveness. So what? The findings will inform a training program for WWWT nurse facilitators and the protocol for a cluster randomised control trial (RCT) to test the effectiveness of WWWT in standard postnatal care.