Proving Eligability for a .com.au Domain
auDA (the policy authority and industry self-regulatory body for the .au domain space) have created a set of eligability rules and policies to control who can register .au domain names, to ensure that copyright, trademark and fair use are respected.
Effectivley it means that to register a .com.au domain name the domain name must be a "close and substanial" match to your Australian registered Trading, Association or Trade Mark name.
When you try to register a .au domain name you will be asked to provife your ABN, ACN or your Incorporated Association Number. Other regitration numbers can also be used. These numbers are then looked up in the appropriate databases to ensure that your chossen domain name is allowable.
This lookup and check is also the reason that .au domains take longer than .com domains to register, and also the reason that .au domains can not always be registered in one business day.
Should the registration organisation have any questions regarding your eligabilty they will contact you to clarify the situation - this is why it is paramount you renter true and usable contact details when you register your domain.