Options for non-citizens:

In exceptional circumstances, if a position cannot be filled by an Australian citizen the citizenship requirement may be waived and applications may be accepted from:

• Permanent residents who can prove they have applied for citizenship

• Permanent residents who are ineligible to apply for Australian citizenship as long as they are prepared to apply for citizenship within 90 days of commencing service.

• Overseas applicants with relevant military experience (see below).

Please be aware that if, as a permanent resident, you refuse citizenship or fail in your application, your ADF service will be terminated.

https://www.defencejobs.gov.au/joining/can-i-join/citizenship

By all means have a crack, but don't make plans revolving around getting in.