The Verdict: Need More Info
Your route depends on what ended your citizenship. Apply to Home Affairs for evidence of Australian citizenship; the response records whether it ended and on what basis.
Most people in this position lost it automatically under the old section 17, which until 4 April 2002 ended the citizenship of any adult who voluntarily took another nationality, without notice. If that is your case, or you lost it as a child when a parent did, you can apply to resume it, with no residence requirement or time limit.
Two other causes work differently. After a formal renunciation, resumption depends on why you renounced. A revocation by the Minister is not covered by resumption at all.