To continue studying in Australia, you must apply for a new Student visa.
If your Student visa will expire before your graduation you might be eligible for a Visitor visa (subclass 600). You will need a letter from ITHEA with the date of your graduation.
If you hold or held a student visa and completed specific qualifications in Australia in the last six months, you might be eligible for a Temporary Graduate visa (subclass 485).
If you want to stay longer for other reasons, you should apply for a new visa that suits your circumstances. Explore your visa options.