@nbayley
You should indicate that you have a student loan on your Tax File Number (TFN) declaration form as soon as you incur the HECS/HELP.
The repayment of your HECS/HELP debt is income-based, not dependent on when you finish your studies. You are required to start repaying your loan once your repayment income exceeds the minimum repayment threshold.
For the 2024-25 income year, the compulsory repayment threshold will be $54,435. If your income is above this threshold, you will need to make repayments regardless of whether you are still studying or have completed your course.
Your employer will start withholding money from your salary to cover your HECS/HELP repayments once you've indicated on your TFN declaration that you have a student loan.