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For what trade do you want to join the Air Force? "Wanting" and "Getting" are two very different things if you want to be a pilot.
How long have you been living in Australia? If you've been here for four years, you will be eligible to apply, this was taken directly from their website:
Do I have to be an Australian Citizen?
For full time or part time entry, citizenship requirements will be satisfied if you are an Australian citizen or a permanent resident eligible for Australian citizenship.