Co-occurring mental illness and substance use disorder, known as dual diagnosis, is a significant challenge to mental health services. Few older adult specific alcohol and other drug treatment services exist, meaning older adult mental health services may become the default treatment option for many. Evidence suggests that dual diagnosis leads to substandard treatment outcomes, including higher rates of psychiatric relapse, higher costs of care and poorer treatment engagement. This paper explores the prevalence of co-occurring alcohol and other drug (AOD) use in an older adult community mental health service in inner Melbourne, Australia. This aim was accomplished by using a retrospective file audit of clinical intake assessments (n = 593) performed on consumers presenting to the service over a two-year period, June 2012-2014. Of consumers presenting to the service, 15.5% (n = 92) were assessed by clinicians as having co-occurring AOD use. Depression predominated in the dual diagnosis group as the primary mental health disorder. Dual diagnosis consumers in this sample were statistically more likely to be male and younger than their non-dual diagnosis counterparts. A limitation of this audit was the lack of implementation of screening tools, leaving assessment to clinical judgement or the interest of the clinician. This may also explain the discrepancy between the results of this study and previous work. Although appearing to be a relatively small percentage of assessments, the results accounted for 92 individuals with complex mental health, AOD and medical issues. Poor screening procedures in a population that is traditionally difficult to assess need to be rectified to meet the future challenges inherent in the ageing baby boomer generation, changing drug use trends and extended lifespans through harm reduction initiatives and medical advancements.