Late Payment Interest in Australia: How to Charge It Legally

Late payments can stall your cash flow and add real pressure to a small business. Adding late payment interest to your invoices can encourage on‑time payment and help you recover the cost of chasing debt - but it has to be done properly and legally in Australia.

In this guide, we’ll step through when late payment interest is allowed, what “reasonable” looks like, how to set it up in your contracts, and practical steps to enforce it without damaging customer relationships.

Our goal is to help you protect your cash flow while staying compliant with Australian Consumer Law and unfair contract term rules - so you can focus on running and growing your business.

What Is Late Payment Interest, And Is It Legal To Charge It?

Late payment interest is an interest charge that accrues on an unpaid invoice after the due date. It’s a standard commercial tool to compensate you for the time value of money and the admin cost of chasing overdue accounts.

In Australia, charging late payment interest is generally legal - provided the customer agreed to it in your contract or trading terms before the goods or services were supplied. You usually cannot add interest unilaterally after issuing an invoice that had no such term.

If you sell to consumers, the interest clause must also comply with the Australian Consumer Law (ACL) and the unfair contract terms regime. Even in B2B, standard form contracts with small businesses are captured by the unfair contract terms laws. The key is transparency and reasonableness.

For more detail on when interest and admin fees cross the line into penalties, it’s worth looking at how the law treats charging late fees and broader rules around late payment fees.

How Do I Set Late Payment Interest Up The Right Way?

To charge interest confidently, build it into your customer contracts and onboarding process. Here’s a practical roadmap.

1) Put It In Your Terms (And Get Acceptance)

Your starting point is clear, written terms that customers accept before you supply anything. Many businesses roll this into their Terms of Trade or a service agreement.

Make sure your interest clause covers:

• The interest rate (expressed as an annual percentage rate, e.g. 10% p.a.).
• When it starts (typically from the day after the due date) and when it stops (on full payment).
• How it accrues (daily accrual is common; state whether it’s simple or compounding).
• Any admin or collection costs you can recover, and the right to suspend services for non‑payment.

Online? Ensure customers tick to accept your terms at checkout or sign a proposal that incorporates them. Offline? Add the terms to your credit application or order form and have them signed.

2) For Account Customers, Use A Credit Application

If you offer 7-30 day accounts, include late payment interest in your Credit Application Terms. This centralises your credit approval process and lets you add related protections like personal guarantees and collection cost indemnities.

3) Choose A Reasonable Interest Rate

There’s no single national cap for commercial late payment interest. The rate must not be “extravagant or unconscionable” (which could make it an unenforceable penalty), and it must be transparent.

Many Australian small businesses use a rate in the single or low double digits per annum (for example, 8-15% p.a.), accruing daily after the due date. The right number for you depends on your industry, typical invoice size, and how quickly you want to nudge payment without harming goodwill.

4) Be Clear About Fees And Costs

In addition to interest, businesses often include a modest admin fee for late accounts and a right to recover reasonable debt collection costs. Keep any fixed fees proportionate, explain them upfront, and avoid anything that looks punitive.

5) Update Your Invoices And Processes

Include the due date, the interest rate and a brief note that interest accrues on overdue amounts under your terms. Build a reminder sequence - a friendly reminder a few days before due date, a prompt on the day, and a clear notice after the due date that interest is accruing under the contract.

What Should A Late Payment Interest Clause Include?

Here are the core elements we’d typically include for a small business in Australia. Tailor them to fit your operations.

• Interest Rate: An annual rate, with daily accrual (e.g., “10% p.a. calculated daily from the day after the due date until paid”).
• Trigger: “If you fail to pay any amount by the due date…” (tie it to unpaid amounts, not to unrelated breaches).
• Compounding: State whether interest compounds monthly/annually or is simple interest. Simple is easier to follow; compounding should be used carefully.
• Recovery Of Costs: A right to recover reasonable legal and collection costs on an indemnity basis.
• Suspension/Withholding: The right to suspend services or withhold delivery for overdue accounts (subject to any essential services obligations in your industry).
• Set‑Off Prohibited (Optional): Prevent the customer from withholding payment by alleging unrelated credits - if appropriate for your model.
• Application Of Payments: Clarify that payments are applied first to costs, then interest, then principal (if you want that order).

These are small choices that have big effects on how quickly you can resolve overdue balances. If in doubt, have a lawyer tailor the terms so they’re strong but still fair under the ACL’s unfair contract terms regime.

How Does The Unfair Contract Terms Regime Affect Late Payment Interest?

Unfair contract terms laws now apply broadly to standard form contracts with consumers and small businesses. If your standard terms go too far - for example, by allowing you to impose excessive fees without transparency or by applying one‑sided rights - the terms could be void and you could face penalties.

Keep your interest and fee terms balanced and clear. Use plain English, highlight key financial terms, and provide a real opportunity for customers to review them before agreeing. In practice, this also reduces disputes: customers are much less likely to push back on charges they understood upfront.

Enforcement: Practical Steps When An Invoice Is Overdue

Even with airtight terms, you’ll want a simple, respectful process for handling overdue invoices. Here’s a common, effective workflow.

Step 1: Send A Friendly Reminder

Sometimes life gets in the way. A polite reminder one or two days after the due date often resolves the issue quickly. Reference the invoice number, the amount outstanding, and note that interest is now accruing under your contract.

Step 2: Send A Formal Overdue Notice

If there’s no response, escalate to a formal notice that sets a clear deadline (e.g. 7 days). Include the interest calculation to date, your bank details, and a contact person to resolve queries.

Step 3: Consider Account Suspension

Subject to your terms and any industry restrictions, suspend services or hold further deliveries until the account is brought up to date. This is a strong incentive that keeps you in control without immediately escalating to legal action.

Step 4: Negotiate A Short Payment Plan (If Needed)

If the client is cooperative but cash‑constrained, a short, written plan can secure payment and preserve the relationship. Keep it tight (weeks, not months), keep future deliveries on cash‑on‑delivery terms, and continue interest under your contract.

Step 5: Escalate To Collections Or Legal

If payment isn’t forthcoming, move to collections or seek legal options. A tailored Debt Collection Agreement sets clear rules with an agency and protects your brand while you recover what’s owed.

Should I Use Security To Back Up My Payment Terms?

For higher‑value accounts or regular trade credit, it’s smart to pair your interest clause with security that improves your recovery prospects if things go wrong.

• General Security Agreement (GSA): A GSA gives you security over the debtor’s personal property. With a General Security Agreement in place, you can register your interest and rank ahead of unsecured creditors.
• PPSR Registration: Registering on the Personal Property Securities Register (PPSR) is critical. Without it, a liquidator or another secured creditor could outrank you. You can register a security interest to protect your position and learn more about what the PPSR is and why it matters.
• Retention Of Title: If you supply goods, retention of title clauses can help you reclaim goods not yet paid for (or their proceeds) - but you still need timely PPSR registration to be enforceable against third parties.
• Director/Personal Guarantees: For companies with limited assets, a personal guarantee from a director, obtained through your Credit Application Terms, gives you another avenue to recover unpaid amounts.

Security doesn’t replace interest clauses - it complements them by improving your leverage and recovery if a customer goes insolvent or disappears.

How Do I Calculate And Apply Late Payment Interest?

The most common approach is a simple daily accrual at an annual rate. Here’s how it works in practice.

Simple Daily Interest Formula

Interest = Outstanding Amount × Annual Rate × (Number of days overdue ÷ 365).

Example: $5,000 invoice, 10% p.a., 20 days late.

Interest = $5,000 × 0.10 × (20/365) = ~$27.40.

Even small amounts add up if an invoice sits for months, which is why simply mentioning the daily accrual in your reminder can spur payment.

Compounding Interest

You can specify monthly or annual compounding. If you do, spell it out in the terms and on your invoices. Compounding increases the total payable and can be more contentious, so weigh the commercial benefit against the relationship impact.

Applying Payments

Your contract can set an “application of payments” order: first to costs, then interest, then principal. This ensures your interest and costs are cleared so the account doesn’t quickly fall behind again. If you use this, keep it clear and fair to avoid unfair contract term concerns.

Common Questions About Late Payment Interest (Answered)

Can I charge late payment interest if it’s not in my contract?

Usually no. You need the customer’s agreement to impose interest. The exception is where a specific statute applies to your industry or you get a court order for interest, but day‑to‑day trade relies on your written terms.

What interest rate is considered “reasonable”?

There’s no fixed cap. A single or low double‑digit annual rate is common in Australia. Focus on transparency and proportionality. Excessive rates or fees risk being struck down as penalties or unfair terms.

Can I also charge a late fee?

Yes, if it’s reasonable and clearly disclosed upfront. A small admin fee plus interest can be appropriate. Avoid “punitive” amounts. If you’re unsure where the line is, review your approach to late payment fees and ensure they’re structured fairly.

Do I add GST to interest?

Interest is generally treated as an input‑taxed financial supply rather than subject to GST. However, tax treatment can be complex - check this with your accountant to ensure your invoicing and BAS are correct.

Can customers set off amounts they say we owe them?

They may try. You can include a “no set‑off” clause so disputed credits don’t block payment of undisputed invoices. Use this thoughtfully so it’s not unfair in standard form contracts. If set‑off is an issue in your industry, have your lawyer draft a balanced clause that aligns with best practice.

Best‑Practice Tips To Reduce Late Payments (Beyond Interest)

Late payment interest works best as part of a broader credit and collections system. A few small changes can transform your cash flow.

• Screen Customers: Use credit checks for larger accounts. Make approvals formal through your Credit Application Terms.
• Shorter Terms For New Accounts: Start with 7-14 days or payment on delivery, then extend once trust is built.
• Clear Proposals and POs: Ensure the purchase order or quote acceptance incorporates your Terms of Trade so there’s no argument about what applies.
• Automated Reminders: Schedule reminders before and after due dates. Keep the tone firm but polite, and note that interest is accruing.
• Security For Larger Deals: Use a General Security Agreement and timely PPSR registration to secure your position, especially if you supply valuable goods on credit. You can register a security interest quickly and understand exactly what the PPSR covers.
• Consistent Escalation: Train your team on when to suspend supply and when to escalate to collections, backed by a robust Debt Collection Agreement.

Key Takeaways

• Charging late payment interest is legal in Australia when it’s clearly agreed upfront, reasonable, and compliant with unfair contract terms laws.
• Build the interest clause into your customer contracts or Terms of Trade, and make acceptance part of your onboarding or checkout process.
• Set a transparent annual rate, state when interest starts and how it accrues, and outline recovery of reasonable collection costs.
• Use structured reminders, the right to suspend services, and, where appropriate, security such as a General Security Agreement with a PPSR registration.
• Keep your terms balanced and easy to understand - this helps with compliance under the ACL and reduces customer pushback.
• Treat interest as one part of a broader credit control system that includes screening customers, tighter payment terms, and a clear escalation pathway.

If you’d like a consultation on setting up late payment interest and strong payment terms for your business, you can reach us at 1800 730 617 or team@sprintlaw.com.au for a free, no‑obligations chat.