The Verdict: Need More Info
This turns on whether you were ever an Australian citizen. Apply to Home Affairs for evidence of Australian citizenship, which will say whether you were and whether it ended.
People are often unsure. Birth in Australia before 20 August 1986 made you a citizen whatever your parents' status, so people who left as small children are often citizens without knowing. An Australian parent does not make you one automatically, because citizenship by descent has to be applied for. An old Australian passport is close to proof that you were.
The answer puts you in one of three places. If you are still a citizen, you need only the certificate. If you were and it ended, you can apply to resume it. If you never were, look at citizenship by descent if a parent was Australian when you were born.