What is penalty interest?

Penalty interest compensates a creditor for the lost time value of money, deters deliberate late payment or strategic delay, and reflects the administrative and enforcement cost of chasing overdue sums. It commonly appears on tax debts, commercial invoices, statutory fines and court judgments. Revenue offices charge it on late tax and penalty debts, suppliers and lenders charge contractual default interest where the contract permits, and courts award pre-judgment and post-judgment interest once a debt is litigated.

Timing follows the legal basis. Contractual default interest usually starts on the due date in the contract, statutory interest often runs from the day after the debt falls due, pre-judgment interest runs from the date of loss or default to judgment, and post-judgment interest runs from judgment until payment. Loan agreements, supply contracts and leases are where contractual clauses usually sit.

A penalty interest rate may be fixed, tied to a benchmark such as the cash rate plus a margin, or updated periodically by statute or administrative notice. Courts and revenue offices publish current rates and effective dates on their websites: court registry pages for pre- and post-judgment rates, state revenue office pages for tax penalty interest and remission policy, and the ATO for the general interest charge, which is worked out daily on a compounding basis, so unpaid interest starts attracting interest itself. Check the effective date before using a rate in a calculation.

Court rates and many state revenue office schemes use simple interest with no compounding, calculated daily as principal x annual rate x days / 365 (or 366 in a leap year). Some commercial contracts specify compounding, and those clauses can be scrutinised for fairness or statutory limits. If a contract sets a rate but a statute imposes a cap or a different rule, the statutory provision may prevail.

Authorities and courts can remit or reduce penalty interest for administrative error by the creditor or revenue office, financial hardship caused by circumstances beyond your control, undue delay by the creditor in bringing proceedings, or payments applied incorrectly. Revenue offices publish remission policies and want an application with evidence, such as financial statements and correspondence showing disputes or payment plans.

To dispute a charge, ask in writing for a breakdown showing principal, rate and its source, dates and formula, check the statute or clause that authorises it, and lodge an objection or review within the time limit, since missing it can forfeit your rights. Unpaid interest can be recovered by assessment, garnishee, execution or insolvency processes. A creditor charging it needs a valid clause or statute, clear terms on invoices and a documented calculation, and for a working capital gap, asset finance or invoice finance may beat aggressive interest charges.