Thanks in advance =)
Interest is charged by the ATO at a rate that is adjusted each quarter. The current interest rate charged is 10.13%. A lot will depend on the size of your debt though as the interest is often automatically remitted if the debt is small.
If the debt you have is larger than the refund, then all of your refund will go to paying the debt off leaving you with a reduced amount to pay back. If the refund you are due is bigger than the debt, then the part that is larger than the debt will be paid to you as the balance of the refund and your debt will be fully paid off.