It’s technically illegal for an Australian citizen to fight for another country IIUC.

No, it’s not illegal to fight in the armed forces of another country, it’s illegal to fight under any other circumstances:

Under Part 5.5 of the Criminal Code, it is an offence to:

• enter a foreign country with an intention to engage in a hostile activity, unless serving in or with the armed forces of the government of a foreign country
• prepare to enter, or for another person to enter, a foreign country with an intention to engage in a hostile activity
• recruit persons to join an organisation engaged in hostile activities, or to serve in or with an armed force in a foreign country.

From this page: ag.gov.au/…/foreign-incursions-and-recruitment-of…

There may be organisations violating the law against recruitment with regards to the IDF, though: …com.au/lone-soldiers-new-australian-idf-recruits…