Background: There is limited Australian research investigating health practitioner knowledge and/or confidence in the diagnosis and treatment of mental health disorders in people with intellectual disabilities (IDs). Method: Ninety-three health practitioners from four professional groups (primary health practitioners, psychologists, mental health practitioners, and alternative therapies) completed a 34-item online survey comprising questions designed to identify participant knowledge of disorders described in the Psychiatric Assessment Schedule for Adults with Developmental Disabilities and the Therapy Confidence Scale-Intellectual Disabilities. Results: Participants demonstrated low knowledge of symptomology and were "moderately confident" to "confident" working with people with IDs. Whilst professional group showed a significant main effect on confidence levels, post-hoc analysis did not detect significant differences between the individual professional groups. Conclusions: Australian participants demonstrated confidence, but low knowledge, in treating individuals with IDs. The development of training to address deficits in practitioner knowledge of symptomology, assessment, and assessment-based communication is recommended.