You would need permission from the Airport Operators to file them as an Alternate, remembering that AUS also has customs/immigration/quarantine restrictions on alternates.
Avalon is a private airport. Canberra can be an alternate, although it may be limited due to capacity.
Gold Coast is the same, and isn't particulally large.
If you think there is ANY chance of not getting in, you'd want to have fuel for a major city airport (SYD, MEL, BNE). Avalon, for example is in non-controlled airspace and has limited ground facilities.
When it comes down to it... you'd need to ask your airline to get approvals to assess and nominate these airports.