The rules
First, it is necessary to understand the eligibility rules introduced by FIFA in 2020. There are two relevant sections of these rules: the section explaining what qualifies a player to represent a country, and the section explaining what allows a player to change association, from one country to another. The first section is simpler than the second. A player must be a citizen of the country they want to represent. In addition to holding citizenship, a player must fulfil one of two requirements: either they, their parents or their grandparents were born in that country, or they have lived in that country for five years continuously. Australia has been both a winner and a loser from these rules in recent years. While Bruno Fornaroli was able to play for the Socceroos due to having lived in Australia continuously for five years, talented young players like Noah Clément (Eintracht Frankfurt) and Jason Vescan-Kodor (Lecce), both Australian citizens, have been prevented from representing Australia due to not fulfilling either the birthright or residence requirements.
The change of association rules are more complex. A player is eligible to change association in several circumstances. The most relevant circumstance for Australian football is the circumstance in which a player, fulfilling both of the above requirements, has represented another country at youth international level in a competitive tournament. Such tournaments include youth World Cups, Olympics, youth confederation cup (eg. underage Euros/Asian Cups) or any tournaments or events which qualify teams for those tournaments. If a player has represented another country in such a tournament, then they are provisionally bound to that country. They can only represent Australia thereafter if they complete a one-time change of association to Australia. This is what Nectar Triantis completed to play for Greece. In contrast, while Alessandro Circati had played for Italy at U20 level, he never represented Italy in a competitive tournament, so he did not need to complete a one-time switch. Cristian Volpato, having represented Italy at competitive youth level, would need to complete a one-time switch.
The other circumstances become more and more obscure and none have arisen in recent years for Australia. A player who has represented a country in a senior competitive international while eligible for another country is eligible to change their association if the following four requirements are fulfilled:
They played no more than three senior internationals for the first country;
They played their last competitive game for that country, at any level, before they turned 21;
They did not play in a World Cup or a confederation tournament (Asian Cup, Euros); and
Three years have passed since they last played a senior international for the first country.
No Australians with Socceroos potential fall into this category, so it is not really relevant. There are other circumstances, but as mentioned, they are obscure and do not seem likely to eventuate.
