Recent studies of autism diagnosis in New Zealand have revealed inconsistent implementation of best practice standards in autism assessment and high rates of parental dissatisfaction with the diagnostic process. These findings have arisen despite the development and dissemination of the New Zealand Autism Spectrum Disorder Guideline (the Guideline). In this study, we investigated current diagnostic processes, and adherence to the Guideline in New Zealand, for an opportunity sample of 117 health professionals who practice across a wide range of diagnostic services, health professionals and geographic regions in New Zealand. The results show considerable variability in practice, and suggest specific areas for improvement. These include the robustness of diagnostic evaluations conducted in private settings, the provision of specialist assessment services for youth and adults presenting for autism diagnosis, and the uniformity of post-diagnostic supports and services. These findings have implications for further implementation of the Guideline in New Zealand.