When to Add Late Fees to an Invoice

Late fees must be in your terms to be enforceable

In Australia, a late fee is only enforceable as a contractual term if it was disclosed to the customer before the transaction. This means your late fee clause must appear in at least one of:

Your credit application (for credit account customers)
Your quote or proposal (before the customer accepts)
Your terms of trade (which should be accepted before or at the time of first supply)
The invoice itself (if the invoice is the first document in the transaction)

Simply adding a note to an overdue invoice that says "late fees now apply" — without prior disclosure — will not create an enforceable obligation. The clause must be agreed to before the debt arose.

What counts as adequate disclosure

The clause does not need to be in large print or on the front page, but it must be reasonably discoverable by the customer. Courts have found that terms buried in an obscure footnote may not be incorporated into the contract. Best practice:

Include the late fee clause in your credit application and have the customer sign it
State the rate prominently in the payment terms section of your quote
Include a short note on each invoice: "Interest at [X]% per month applies to amounts overdue beyond [payment terms]"

Common late fee structures

Flat fee: A fixed dollar amount (e.g. $50 per month overdue). Simple and easy to communicate. Works well for smaller invoice values where a percentage-based fee would be negligible. Common for consumer-facing businesses.
Monthly percentage on the overdue balance: A percentage of the outstanding amount per month (e.g. 2% per month = ~26.8% per annum). Scales with the debt size. More appropriate for B2B and larger invoices. The most common structure for commercial terms of trade.
Daily rate: Some businesses specify a daily interest rate (e.g. 0.067% per day = ~2% per month). This can be precise but is more complex to calculate and explain to customers.

Penalty clauses and the law

Australian courts follow the rule against penalties — a contractual clause that imposes a payment disproportionate to the legitimate interest of the innocent party may be struck down as an unenforceable penalty. The High Court considered this area in Andrews v Australia and New Zealand Banking Group Ltd [2012] HCA 30, affirming that Australian law still recognises the penalty doctrine.

The practical implication: a late fee that is a genuine pre-estimate of your loss from late payment (administrative costs, funding costs) is more defensible than one that is punitive. Rates up to 2–3% per month are generally seen as commercial, but very high rates or flat fees that bear no relationship to actual loss may be challenged. This is a complex area of law — seek legal advice if you are designing a high-value late fee regime.

How to apply a late fee to an overdue invoice

Do not amend the original invoice. Instead:

Issue a new invoice for the late fee amount, referencing the original invoice number and the period for which the fee applies (e.g. "Late fee on Invoice #INV-2026-042 — 30 days overdue")
If you are GST-registered, late fees are generally a taxable supply — include GST on the late fee invoice
Send the late fee invoice with your next payment reminder so the customer can see both amounts outstanding

You can use our free invoice generator to create the late fee invoice. For payment reminder wording at each stage, see the payment reminder templates.

Using a late fee calculator

The Merion late fee calculator lets you calculate the interest accrued on an overdue invoice based on your rate, the principal amount and the number of days overdue. Use it to prepare the exact figure for your late fee invoice.

Practical considerations

Will applying the fee damage a valuable long-term relationship? Consider a courtesy call before applying
Is the customer genuinely disputing the invoice, or just slow? Escalate disputes separately — see the invoice disputes guide
Is the amount worth the friction? For small invoices, the late fee may cost more to enforce than it recovers
For habitually slow payers, consistent application of fees sends a clear signal — inconsistency undermines the deterrent effect

Frequently asked questions

Can I add late fees retroactively?

Generally no. Late fees are only enforceable if they were disclosed in your terms before the original invoice was issued. Adding a late fee clause after the invoice has been sent — or informing the customer of the fee only after it has accrued — is unlikely to be enforceable. Always include your late fee terms in your credit application, quote or invoice from the outset.

What is a reasonable late fee?

There is no prescribed rate under Australian law, but courts may look critically at fees that are out of proportion to the actual cost of delay. Common structures: a flat fee of $25–$50 for consumer-facing businesses; or a monthly interest rate of 1.5–3% on the overdue balance for B2B. For B2B, a rate equivalent to the RBA cash rate plus 2–5 percentage points is often cited as a reasonable benchmark. Whatever you choose, disclose it clearly in your terms.

Do I need to notify the customer before charging late fees?

If your terms of trade already include the late fee clause (and were provided to the customer before the supply), you are not legally required to give separate notification before the fee accrues. However, as a practical matter, it is good practice to remind the customer that late fees will apply when sending your first overdue reminder. This reduces disputes and maintains the relationship.

Will charging late fees damage the customer relationship?

It depends on how you handle it. Applying a late fee immediately on the first day of overdue, without any prior reminder, can feel punitive and damage goodwill. A better approach for relationship-sensitive situations: send a friendly payment reminder first; on the second or third reminder, note that late fees are accruing under your terms; only apply the fee if payment is still not received after your reminder sequence. For habitually slow payers, applying the fee consistently sends a clear message.