This paper reports on the outcomes of an intensive collaborative community-based perinatal and infant mental health (PIMH) Day Program designed to improve the emotional-social wellbeing of mothers with a moderate to severe mental illness and attachment to their infant. Coordinated by the Queensland Centre for Perinatal and Infant Mental Health, Child and Youth Mental Health, Children's Health Queensland Hospital and Health Service, the program was delivered in 2016 to twenty six mother-infant dyads through an interagency partnership between adult mental health, child and youth mental health and community child health across two Hospital and Health Service sites in Queensland, Australia. The study resulted from recommendations of a pilot project conducted in Brisbane in 2009. An infant's developmental outcomes are predicated by their earliest attachment to their caregivers. It is estimated 40% to 60% of children whose parent has a mental illness are at risk to their mental wellbeing. With more than 16% of Australian women and 10% of fathers developing a perinatal mental illness, there is an identified gap in both the availability and flexibility of services to this vulnerable population. Evidence suggests that service delivery to perinatal mothers with complex mental health issues is typically 'siloed' and that treating maternal mental health alone does not improve outcomes for infants. The unique feature of the PIMH Day Program is the collaboration of three separate health services delivering one program to mothers and infants together. Cairns and Hinterland Hospital and Health Service (HHS) and Townsville HHS are two large regional cities located in north Queensland. Clinicians from adult mental health, child and youth mental health and child health from each HHS delivered three (3) six week (one day per week) perinatal and infant mental health programs between May to November 2016 through a collaborative and integrative methodology. Multidisciplinary clinicians from the three services worked together to deliver the program using a dedicated program manual. The intervention resulted in statistically significant improvements in pre-post quantitative measures relating to mother's emotional-social wellbeing and infant attachment, positive feedback from selfreport surveys on program elements, and engagement in post referral pathways. Clinicians reported high level satisfaction with the collaborative process, resource sharing and transfer of knowledge and skills. They support the intervention as a sustainable, effective and efficient model of service. The findings support the premise that interagency collaboration between health services brings about improved mental health outcomes for mothers with a mental illness and enhanced understanding and sensitivity to their infants. The study resulted in statistically significant results in both sites supporting the results of the 2009 study. This program demonstrates that collaboration, partnerships and integration across service boundaries and professional disciplines offers a promising opportunity to deliver a service that supports parenting and an infant's emotional-social development. The Day Program translates across settings while building workforce capacity in perinatal and infant mental health. [ABSTRACT FROM AUTHOR]