Ten to fifteen percent of prisoners in New Zealand have a serious mental illness. Service improvements to this population have been advocated for by the Chief Justice, the Ombudsman's Office and the Auditor-General. The Northern regional forensic mental health services are developing a 'best practice' model of care for service delivery by prison in-reach teams to be implemented in 6 prisons. The research aim is to evaluate the effectiveness and efficiency of this model to improve mental health and psychosocial outcomes for prisoners with serious mental illness. A mixed-methods, pre- and post-test research evaluation will be used to compare detection rates, service response, mental health outcomes, post-release community mental health engagement and reoffending rates, before and after implementation of the model. The evaluation will include a financial analysis of cost effectiveness. Successful evaluation may provide a benchmark for the national implementation of an improved model of care.