What are the requirements for foreigners to join the RAAF as a fighter pilot?
Only citizens of a country can apply in the government jobs (including armed forces) of that country. There can be some exceptions but I hardly doubt that a foreigner can be hired or trained for any position in military which has access to classified information. Being a fighter pilot even has more restrictions.

Royal Australian Air Force does not hire any foreigner:


  Can I apply if I am not an Australia Citizen?
  
  To be eligible to join the Air Force, you must be an Australian
  Citizen.


According to Recruitment Centre of Australian Defence Force, this page lists the proofs required for citizenship.