BACKGROUND: Perinatal mental health has pervasive impacts on the wellbeing of both the mother and child, affecting quality of life, bonding and attachment and cognitive development. OBJECTIVES: The aim of this study was to (i) quantify the costs to government healthcare funders, private health insurers and individuals through out-of-pocket fees, of women with postnatal major depressive disorder (MDD); and (ii) identify any socioeconomic inequalities in health service use and costs amongst these women. METHODS: A whole-of-population linked administrative dataset containing the clinical records and health service use for all births in the state of Queensland, Australia between 01 July 2012 and 30 June 2015 was used (n = 189,081). Postnatal MDD was classified according to ICD-10 code, with women hospitalised for MDD in the 12 months after birth classified as having 'postnatal MDD' (n = 728). Health service use and costs from birth to 12 months post-birth were included. Total costs included cost to government funders and private health insurers and out-of-pocket fees. Total costs and costs to different funders were compared for women with postnatal MDD and for women without an inpatient event for postnatal MDD, with unadjusted means presented. A generalised linear model was used to compare the difference in total costs, adjusting for key confounders. Costs to different funders and number of different services accessed were then compared for women with postnatal MDD by socioeconomic status, with unadjusted means presented. RESULTS: The total costs from birth to 12 months post-birth were 636% higher for women with postnatal MDD than women without an inpatient event for postnatal MDD, after accounting for differences in private hospital use, mode of birth, clinical characteristics and socioeconomic status. Amongst women with postnatal MDD, the cost of all services accessed was higher for women of highest socioeconomic status than for women of lowest socioeconomic status (A$15,787.66 vs A$11,916.94). The cost of services for women of highest socioeconomic status was higher for private health insurers (A$8941.25 vs A$2555.26), but lower for public hospital funders (A$2423.39 vs A$6582.09) relative to women of lowest socioeconomic status. Outside of public hospitals, costs to government funders was higher for women of highest socioeconomic status (A$2766.80 vs A$1952.00). Women of highest socioeconomic status accessed more inpatient (8.2 vs 3.1) and specialist services (13.4 vs 5.5) and a higher proportion had access to psychiatric specialist care (39.7% vs 13.6%) and antidepressants (97.6% vs 93.8%). CONCLUSION: MDD is costly to all funders of healthcare. Amongst women with MDD, there are large differences in the types of services accessed and costs to different funders based on socioeconomic status. There may be significant financial and structural barriers preventing equal access to care for women with postnatal MDD.