Background: Previous estimates from 2007 found that co-occurring mental and/or substance use disorders were a pervasive feature of Australia’s mental health. Since that time there have been shifts and improvements in the conceptualisation and incorporation of co-occurring disorders in research and treatment settings. The current study provides up-to-date estimates on the prevalence of co-occurring mental and/or substance use disorders, highlights common patterns of co-occurrence, identifies significant correlates and examines any changes in the extent of co-occurring disorders since 2007. Methods: Data were from the two Australian National Surveys of Mental Health and Wellbeing conducted in 2020–2022 ( N = 15,893) and 2007 ( N = 8841). Descriptive statistics were estimated for the number of co-occurring conditions, correlations and pairwise conditional probabilities. Multinomial logistic and robust Poisson regressions were used to identify significant correlates and compare changes in co-occurring conditions across surveys. Results: Approximately 46% of people with a mental or substance use disorder in the past 12 months experienced two or more diagnosable conditions. There was little evidence to suggest that the prevalence of co-occurring disorders has changed since 2007 (Prevalence Ratio (PR) = 1.08, 95% CI = 0.98–1.18). Subgroup analysis indicated that those aged 16–24 years were significantly more likely to experience any co-occurrence in 2020–2022 compared with those aged 16–24 years in 2007 (PR = 1.44, 95% CI = 1.17–1.77). Conclusions: Co-occurring mental and substance use disorders remain endemic in Australia. Indeed, they appear to be increasingly problematic in younger, more recent cohorts. The results suggest that continued effort is needed to develop and implement transdiagnostic interventions that target broad contextual and/or societal factors.
