Paul...
Here are a couple of  quotes  from the Department of Fair Trading (NSW)
regarding Registering a Business Name

1. "The Department does not check to see if a trade mark is registered
similar to the name for which you apply. Responsibility is not accepted
for registration of a name which might infringe proprietary rights
acquired by registering a trade mark"

This is certainly an area that can get you into trouble....

And most relevant to your concerns over generic ownership of names
*.com.au etc...
2. " If you register a name consisting of generic words another name may
be registered containing those same words prefixed or suffixed by a
distinguishing word or words, e.g. registration of the name "Medical
Centre" would not preclude the later registration of the name "Mid Coast
Medical Centre"."

As you need to give a RBN or ACN to get your *.com.au domain name, the
cross checks (or lack of them) made by the ASC and/or the Dept. of Fair
Trading in each State is the base line from which legal status of any
'protection' or 'ownership' of a business  stems. It seems to me that
this is at least a basic protection for the *.com.au position... This is
by comparison to getting a *.com; When I got my *.com all I had to do
was state my name and see if it was already taken plus claim that I
needed it to further my core business directions and that was it... no
ACN no RBN nothing else was required... N.B. the Australian Name
Registration system also works for the register  Incorporation's and
Association names, so  the same ASC checks and requirements for non
duplication are made, so it should work for *.org.au etc... But there
are many more loop holes in this area if you ask me...

Cheryl