One in ten fathers experience mental health difficulties in the first year postpartum. Unsupportive job conditions that exacerbate work-family conflict are a potential risk to fathers' mental health given that most new fathers (95%) combine parenting with paid work. However, few studies have examined work-family conflict and mental health for postpartum fathers specifically. The aim of the present study was to identify the particular work characteristics (e.g., work hours per week, job quality) associated with work-family conflict and enrichment, and fathers' mental health in the postpartum period. Survey data from 3243 fathers of infants (aged 6-12 months) participating in the Longitudinal Study of Australian Children were analysed via path analysis, considering key confounders (age, education, income, maternal employment, maternal mental health and relationship quality). Long and inflexible work hours, night shift, job insecurity, a lack of autonomy and more children in the household were associated with increased work-family conflict, and this was in turn associated with increased distress. Job security, autonomy, and being in a more prestigious occupation were positively associated with work-family enrichment and better mental health. These findings from a nationally representative sample of Australian fathers contribute novel evidence that employment characteristics, via work-family conflict and work-family enrichment, are key determinants of fathers' postnatal mental health, independent from established risk factors. Findings will inform the provision of specific 'family-friendly' conditions protective for fathers during this critical stage in the family life-cycle, with implications for their wellbeing and that of their families.