You will not be able to register the domain through DreamHost (I think you are already aware of this), but you should have no problems hosting it here.
The procedure is exactly the same as hosting a .com that is registered elsewhere. First, you simply use whatever configuration utility your registrar provides to configure the DNS records for the domain to point at the DreamHost DNS servers.
Then you add the domain to the DreamHost hosting system using the panel.
Domains -> Manage Domains -> Add New Domain / Sub-Domain
You add the domain as 'Fully Hosted'.
Give it a day or two for the DNS changes to fully propagate around the Internet and your .au domain should work fine.
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