Maternal substance use (SUD) and mental health disorders commonly co-occur and require complex treatment. Information on women’s use of appropriate services in the perinatal period is limited. Data from the New Zealand Infant Development, Environment and Lifestyle Study were used to examine the characteristics of women with high probability of substance use and/or psychiatric disorder and rates of service use at 1 and 12 months following birth (n = 221). The Substance Abuse Subtle Screening Inventory-3 and the Brief Symptom Inventory were used to identify risk of disorder. Despite a high proportion of mothers with disorder risk, rates of specialist treatment remained low across SUD and psychiatric groups at 1 (27–39%) and 12 months postnatal (25–42%). Very low rates of women with comorbid disorder received both mental health and substance use treatments (1 month, 4.5%; 12 months, 7.3%). There was no association between service use and risk for psychiatric disorder at 12 months after birth. The findings suggest that even when services are publicly funded, they may be under-utilised or under-resourced to provide effective treatment, despite the high and complex needs of this population.