You can do all these things using category settings and custom groups.
There are several topics that deal with these configurations, just use internal research
Basically you can create an “Audi Australia” group and you can configure the group so that users can enter only if approved by the owners of the group or only if users register with a particular email domain (e.g. audi.australia@your.site.com).
After that you need to create an “Audi Australia” category and you have to configure the permissions to “create see and reply” only for the “Audi Australia” group.
No one outside the group “Audi Australia” will see that category or see the topics that belong to that category (except for the staff of course).
All the categories with the “everyone can create reply and see” permissions will be public