As an Australian who lives in our national capital, I can assure you that our friendly local FVEY agency will happily employ Australian citizens at a Top Secret

As an Australian who lives in our national capital, I can assure you that our friendly local FVEY agency will happily employ Australian citizens at a Top Secret Positive Vetting level who are naturalised immigrants or dual citizens (a huge chunk of this country are dual British citizens, in particular). I get the impression that it makes your life a bit more difficult, but it's definitely achievable.