It is an automatic acquisition if you are born in Australia and have a parent who is an Australian citizen or a permanent resident at the time of your birth.
You are also automatically an Australian citizen if you are born in Australia and you are an ordinarily resident in Australia for a period of 10 years beginning on the day you are born.

but one of your parents at the time of your birth as an Australian citizen, you may be eligible to apply for the Australian Citizenship.

You may apply for an Australian citizenship if your adoption was completed outside Australia by an Australian citizen under the Hague Convention or Bilateral arrangement.

For many migrants the most common way to citizenship is Citizenship by conferral. Granting citizenship by conferral happens when you are a Permanent Resident of Australia or a New Zealand citizen and you meet specific eligibility requirements and pass the citizenship test.