Objective To determine, using face-to-face diagnostic interviews, the prevalence of common mental disorders (CMD) in a cohort of adult Indigenous Australians, the cultural acceptability of the interviews, the rates of comorbid CMD and concordance with psychiatrists' diagnoses. Design Cross-sectional study July 2014-November 2016. Psychologists conducted Structured Clinical Interviews for Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision Axis I Disorders (SCID-I) (n=544). Psychiatrists interviewed a subsample (n=78). Setting Four Aboriginal Medical Services and the general community located in urban, regional and remote areas of Southern Queensland and two Aboriginal Reserves located in New South Wales. Participants Indigenous Australian adults. Outcome measures Cultural acceptability of SCID-I interviews, standardised rates of CMD, comorbid CMD and concordance with psychiatrist diagnoses. Results Participants reported that the SCID-I interviews were generally culturally acceptable. Standardised rates (95% CI) of current mood, anxiety, substance use and any mental disorder were 16.2% (12.2% to 20.2%), 29.2% (24.2% to 34.1%), 12.4% (8.8% to 16.1%) and 42.2% (38.8% to 47.7%), respectively-6.7-fold, 3.8-fold, 6.9-fold and 4.2-fold higher, respectively, than those of the Australian population. Differences between this Indigenous cohort and the Australian population were less marked for 12-month (2.4-fold) and lifetime prevalence (1.3-fold). Comorbid mental disorder was threefold to fourfold higher. In subgroups living on traditional lands in Indigenous reserves and in remote areas, the rate was half that of those living in mainstream communities. Moderate-to-good concordance with psychiatrist diagnoses was found. Conclusions The prevalence of current CMD in this Indigenous population is substantially higher than previous estimates. The lower relative rates of non-current disorders are consistent with underdiagnosis of previous events. The lower rates among Reserve and remote area residents point to the importance of Indigenous peoples' connection to their traditional lands and culture, and a potentially important protective factor. A larger study with random sampling is required to determine the population prevalence of CMD in Indigenous Australians.