Your result
Australia — Reacquiring Lost Citizenship
Possible
Whether this route is open to you depends on something you can establish in a single step: whether you were ever an Australian citizen in your own right.
People are genuinely unsure about this more often than it sounds. Being born in Australia before 20 August 1986 conferred citizenship automatically, whatever your parents' status, so people who left as small children are frequently citizens without knowing. Having an Australian parent does not by itself make you one — citizenship by descent has to be applied for and granted, so it is possible to have been eligible your whole life and never to have held it. And holding an Australian passport at some point in the past is close to conclusive that you did hold it.
Apply to the Department of Home Affairs for evidence of Australian citizenship. The result tells you which of three situations you are in: you are a citizen and need only the certificate; you were a citizen and it ceased, in which case resumption is open with no residence requirement and no deadline; or you never held it, in which case citizenship by descent is the thing to look at if a parent of yours was Australian when you were born.