Your employer withholds tax from your salary (called PAYG - Pay As You Go Withholding) on the basic Australian tax tables.
If you ticked that you have a HELP debt - they'll withhold an extra 1% ($48k) to 10% ($151k+) from your salary - to try to 'match' the required HELP debt repayment required when you lodge your tax return.
Say you earn $155k = that is $15,500 extra they'll withhold.
But say you only have nothing or maybe a smaller amount (say 3k) left on your help debt - you can tell them and going forward they won't withhold extra for HELP debt (and the extra will go into your pocket).
If you want forced savings - you can tell your employer to split your salary into more than one account.
Or simply set up an automatic transfer so you put the "extra" funds into savings.
Or of course - put it in your offset account or make an extra repayment from your mortgage.