Objectives: To describe the frequency, type and quality of mental health treatment among Australian adults with past-year affective and/or anxiety disorders. Design, setting and participants: Retrospective analysis of data for 8831 adults aged 16–85 years interviewed for the 2007 National Survey of Mental Health and Wellbeing, of whom 17% (n = 1517) met International Classification of Diseases, 10th revision (ICD-10) criteria for a past-year affective and/or anxiety disorder. Main outcome measures: Three levels of mental health treatment received in the past year: (1) any consultation with a health professional for mental health; (2) any evidence-based intervention (antidepressant medication, mood stabiliser medication, cognitive behaviour therapy and/or psychotherapy); and (3) minimally adequate treatment (a “dose” of an evidence-based intervention above a minimum threshold, consistent with treatment guidelines). Results: Of participants with past-year affective and/or anxiety disorders, 39% sought professional help for mental health, 26% received an evidence-based treatment, and 16% received minimally adequate treatment. After controlling for clinical factors including type and severity of disorder, the odds of all levels of treatment were lower among younger adults (16–29 years) compared with middle-aged adults, and the odds of receiving an evidence-based treatment or minimally adequate treatment were lower among people who consulted a general practitioner only compared with a mental health professional. Conclusions: Closing the gap in treatment quality requires strategies to increase the use of evidence-based interventions, and to ensure these are delivered in sufficient doses. Research to elucidate why some patients are at increased risk of inadequate treatment, and the aspects of treatment that contribute to inadequate care, is indicated.