Typically, the way to get assessed for Autism is through two professional diagnostic assessments.
One from a clinical psychologist.
The other from an Allied Health Professional, such as Occupational Therapist or Speech Pathologist.
This can be done privately, be prepared to pay a lot of money. Last I looked into cost about $1800
You can be put on the Government Allied Health waitlist, which last I looked into it had a wait period of 3 years.
Disclaimer: at the time of writing this information, all contents were true and correct based on personal lived experience. Please note that is a guide only and over time they may change their processes and some information contained in this article may become void and irrelevant. If in doubt, please go directly to the source, NDIS to seek clarity and clarification.