Medibank and Medicare both have a significant differences.
Medibank is a private health insurance provider. Before coming to Australia, a person need a medical insurance to satisfy their visa if they intend to stay for a long period of time for example for study or migration.
You can read more about Medibank here : https://www.medibank.com.au/
You can read more information here and make decision which private health insurance is the best for you : https://www.comparingexpert.com.au/health-insurance/companies/
Medicare is the Australia’s universal insurance scheme. However, Medicare will apply mostly to Australia or New Zealand citizen, permanent resident and certain visitors under certain circumstances.
You can read more information here : https://www.comparingexpert.com.au/health-insurance/companies/
or if you are interested to know more about Medicare you can find it here :