Short answer
If you finish your main course early, your enrolment may end earlier than expected. Home Affairs generally allows up to three months after early completion before considering visa cancellation for this issue.
If you finish your main course earlier than the end date on your CoE, your CoE may end and you may no longer be enrolled. This can put your Student visa at risk.
Home Affairs generally allows students up to three months in Australia after early course completion. It generally would not consider cancelling the visa for this issue during that period.
If you plan to stay in Australia for more than three months after finishing early, you may need to apply for another visa.