Australia - Passport & Nationality - Australian Citizenship by Conferral
A Passport by Conferral is based the Australian citizenship of a parent at the time of your birth, and this citizenship is subsequently lost or given up.
The fact that a parent may have subsequently lost their Australian Citizenship for whatever reason (due, for instance, to taking on another nationality at a time that Australia did not allow Dual Nationality) is irrelevant - what is important was that your parent was Australian at the time of your birth.
A person becomes a citizen on the date they are registered as a citizen by the Department of Immigration and Citizenship (DIAC). A certificate of Australian citizenship is issued at the time of registration.