Resume Australian Citizenship: Fill & Download for Free

As everybody else says, this varies greatly by the country. Australia is remarkably generous here (probably a lot more so than most other countries), and provides a very simplified procedure for the resumption of citizenship:Resume your Australian citizenship - summaryResume Australian citizenship - eligibilityFormer citizens eligible to resume their Australian citizenship include those who have renounced it (e.g. to become a citizen elsewhere, if that country required it), as well as those who have automatically lost their citizenship due to naturalization elsewhere (prior to 2002), or merely due to a long-term residence abroad (prior to 1958).There is no requirement for a former Australian citizenship to obtain Australian permanent residence, or to spend a certain amount of time in Australia, in order to resume one’s Australian citizenship.

To add to Rory Browne ‘s answer, Australia also has a process for “resuming Australian citizenship”, available e.g. to persons who lost Australian citizenship due to naturalizing elsewhere (under the laws in effect prior to 2002), or due to living abroad for a long time (under the laws in effect in 1948–1958):https://immi.homeaffairs.gov.au/citizenship/become-a-citizen/become-citizen-again

Renouncing citizenship is never something you should take lightly.That said. Australia have a page dedicated to people in your situation:Become an Australian citizen again (resuming Australian citizenship)How your Australian citizenship ceased may affect your eligibility.You are eligible to resume citizenship if you:became a citizen of another country as an adult before 4 April 2002gave up or renounced your Australian citizenship so that you can acquire or retain the citizenship of another countryrenounced Australian citizenship to avoid significant hardship or detrimentyour Australian citizenship was revoked as a child when your responsible parent's Australian citizenship ceasedwere an Australian citizen by registration or naturalisation and your Australian citizenship ceased because you resided outside of Australia and New Guinea continuously for 7 years between 26 January 1948 and 8 October 1958You are not eligible to resume Australian citizenship if:you were born in Papua and lost your Australian citizenship automatically on 16 September 1975 under Papua and New Guinea law (when Papua New Guinea achieved independence)your Australian citizenship ceased for a reason not already mentioned