There is no definitive answer to this question as it depends on a number of factors, such as your age, whether you are studying or working, and your immigration status. However, in general, you can usually stay on your parents' health insurance until you are 30 years old.
If you are studying, you may be able to stay on your parents' health insurance until you finish your course, even if you are over 30.
If you are working, you may be able to stay on your parents' health insurance if your employer does not offer health insurance, or if your health insurance premiums would be too expensive.
If you are an Australian citizen or permanent resident, you may be able to stay on your parents' health insurance even if you are working or studying overseas.