If somehow your VISA expires and you still want to continue study in Australia, you need to apply for a new Student Visa. Do not worry if your VISA expires before you become a Graduate, you can apply for a Visitor VISA i.e. Subclass 600. To stay for your Graduation, the student must have a letter from the Authority of the Educational Institute along with the date of your graduation.
If you’re a mastermind and recently Graduated from a well-recognized Engineering Institute you can make an effort to apply for the Skilled Recognition Graduate VISA i.e. Subclass 476.
Even if you don’t have any of the above-mentioned conditions, you can still explore and try to find a reason that has a solid argument for you to apply for a new VISA.