The Verdict: Need More Info
Whether you are already Australian turns on your parents' status when you were born, or on your first ten years. Apply to Home Affairs for evidence of Australian citizenship to settle it.
Since 20 August 1986, birth in Australia is not enough on its own. Descent does not apply either, because it is for people born outside Australia.
You are a citizen from birth if either parent was an Australian citizen or permanent resident on the day you were born. Permanent residence is enough, which covers many children of migrants who never naturalised.
If neither parent qualified, you became a citizen on your tenth birthday if you lived in Australia for your first ten years, whatever your parents' status. Once evidence of citizenship is issued, you can apply for a passport.