Your result
Australia — Reacquiring Lost Citizenship
Possible
You held Australian citizenship and something ended it, and which route applies depends on what that something was. The good news is that the most common cause is also the most recoverable.
Most people in this position lost their citizenship automatically under the old section 17, which until 4 April 2002 stripped Australian citizenship from any adult citizen who voluntarily acquired another nationality. It happened silently, with no notice and no decision, which is exactly why people cannot say what happened. If that is your case, resumption is available with no residence requirement and no time limit. The same is true if you ceased as a child because a parent's citizenship ceased.
Two other causes behave differently: a formal renunciation you applied for, where resumption depends on why you renounced, and a revocation by the Minister, which resumption does not cover at all.
Apply to the Department of Home Affairs for evidence of Australian citizenship. The response records whether your citizenship ceased and on what basis, which is the fact this whole question turns on.