If you have a debt owing to the ATO, the ATO charge a high interest rate on money owed to them.  It is 9.75% from the date the payment was due to be paid. If you have outstanding Tax Returns or Activity Statements:

The non-lodgement of tax returns and activity statements can cost you penalties.  The ATO charge $175 for every 28 days the form is outstanding from it's original lodgement date.