To be a little more subtle, there is no ban on "all" overseas travel from Australia, as such. There is a ban on Australian residents (i.e. those who normally reside in Australia regardless of nationality) traveling overseas, unless they meet some specific exceptions.

So foreigners, for example, on student visa, who normally live in Australia, would not be permitted to leave Australia, unless, I assume their visa is close to expiry and they are returning permanently overseas.

However, conversely, Australians who normally do live overseas can come and go as they please, subject to restrictions in travelling from their home countries, provided they can deal with 14 day isolation requirements when they arrive in Australia, and potentially when they return to their normal country of residence overseas as well.