Objectives: There is currently no agreed Australian standard for the diagnosis of autism spectrum disorder (ASD) even though there are specific diagnostic services available. We suspected inconsistency in the diagnostic practices of health professionals in Australia and aimed to assess these practices across the nation by surveying all relevant professional groups. Design: In this study, we completed a survey of 173 health professionals whose clinical practice includes participating in the diagnostic process for ASD in Australia. Participants completed an online questionnaire which included questions about their diagnostic setting, diagnostic practice and diagnostic outcomes in 2014–2015. Participants: Participants covered a range of disciplines including paediatrics, psychiatry, psychology, speech pathology and occupational therapy. All states and territories of Australia were represented. Setting: Participants came from a range of service settings which included hospitals, non-governmental organisations, publicly funded diagnostic services and private practice. Results: There was variability in diagnostic practices for ASD in Australia. While some clinicians work within a multidisciplinary assessment team, others practice independently and rarely collaborate with other clinicians to make a diagnostic decision. Only half of the respondents reported that they include a standardised objective assessment tool such as the Autism Diagnostic Observation Schedule in ASD assessments, and one-third indicated that they do not include measures of development, cognition and language in assessments where ASD is suspected. Conclusions: Reported practice of some professionals in Australia may not be consistent with international best practice guidelines for ASD diagnosis. These findings highlight the need for a minimum national standard for ASD diagnosis throughout Australia that ensures best practice regardless of the type of setting in which the service is provided.