You must have student healthcare cover before arriving in Australia and for the duration of time you are in Australia – this is a visa requirement of the Department of Home Affairs. International students undertaking formal studies in Australia, and their dependents (for example, spouses and children under 18 years old), must obtain OSHC.
For further information about OSHC and other optional insurances visit https://www.studyinaustralia.gov.au/English/Live-in-Australia/Insurance