Does the ATO require a specific invoice number format?

No. The ATO requires every invoice to have a unique identifier so it can be referenced unambiguously, but it doesn’t mandate a format. Sequential numbers like INV-0001, INV-0002 are the most common because they’re simple, auditable, and hard to duplicate.

Can I start my invoice numbers at something other than 1?

Yes. Some businesses start at 1001 or even 10001 so their first invoices don’t look obviously like first invoices. The only rule is that each number must be unique — the starting point is your choice.

What happens if I skip an invoice number?

Nothing legally — the ATO doesn’t require a perfectly consecutive sequence, just unique identifiers. But gaps can raise questions during audits. Best practice is to keep a note of any skipped numbers and why (e.g. “INV-0042 voided before send”).

Can I reuse an invoice number?

No. Each invoice number must be unique. Reusing a number creates ambiguity in your records and your client’s, makes BAS reconciliation a nightmare, and is a red flag in an audit. If an invoice was cancelled, issue a credit note and use a fresh number for any replacement.

Should I include the year in my invoice numbers?

Optional but useful for high-volume businesses. A format like 2026-001 makes it instantly clear which year an invoice belongs to and resets the sequence each year so the numbers stay short. For low-volume sole traders, a simple INV-0001 sequence that keeps growing is easier to manage.

How to Number Invoices in Australia (Formats That Work)

Quick answer

Use a simple sequential format like INV-0001, INV-0002, INV-0003. The ATO requires every invoice to have a unique identifier so it can be referenced unambiguously, but it doesn’t mandate a specific format. Pick a system that’s easy to read, hard to duplicate, and auto-incremented — and stick with it.

Why invoice numbers matter

A unique invoice number does four jobs:

Reference: “Please pay invoice INV-0042” is precise; “The invoice from last Tuesday” isn’t.
Audit trail: the ATO can match transactions across your records using the number. Gaps and reuses make audits painful.
BAS reconciliation: when GST is reported quarterly, the invoice number ties each amount back to a specific sale.
Client accounting: your client’s bookkeeper records the number in their system. Duplicates break their books.

The four formats that work

1. Simple sequential (recommended for most)

Format: INV-0001, INV-0002, INV-0003…

The gold standard for sole traders and small freelance businesses. Numbers never reset, so there’s no ambiguity between years. The leading zeros (0001 rather than 1) make the numbers sortable in folders and emails and give room to grow without restructuring.

Use this if you send fewer than ~1,000 invoices a year and don’t need to filter by year visually.

2. Year-prefixed (good for higher volume)

Format: 2026-001, 2026-002… resets to 2027-001 on 1 January 2027.

The year is instantly visible, which speeds up archive lookups. The sequence resets each year, so numbers stay short even at high volume.

Trade-off: you need a system that knows when to reset. If you do this manually, set a reminder on 1 January (or 1 July if you reset on the financial year).

3. Financial-year prefixed (BAS-friendly)

Format: FY25-001 for the 2024–25 financial year, then FY26-001 from 1 July 2025.

Aligns with the Australian financial year (1 July — 30 June), which makes BAS reconciliation and year-end record-keeping easier. Most useful for businesses with accountant-driven workflows.

4. Client-prefixed (rare, but useful sometimes)

Format: ACME-001, ACME-002, STARK-001…

Useful only if you have a small number of major clients and want each client’s invoices to be obviously grouped. Most accounting software treats this as fine because the full string is still globally unique.

Don’t use this if you have many ad-hoc clients — you’ll create new prefixes constantly, and the system gets messy.

What NOT to do

Don’t reuse numbers. Ever. If an invoice was cancelled, issue a credit note (referencing the original number) and use a fresh number for any replacement.
Don’t use a random number generator. “Look unique” isn’t the same as “is unique”. Random numbers also can’t be sorted chronologically.
Don’t use just the date. “2026-06-04” isn’t unique if you send two invoices the same day.
Don’t put sensitive information in the number. Project codes, client tax file numbers, or internal phrases shouldn’t appear — the invoice number is visible to anyone copied on the email.

What if you skip a number?

Nothing legal happens. The ATO requires unique identifiers, not perfectly consecutive ones. But gaps can raise questions during an audit, so the best practice is:

If you create a draft and then delete it before sending, log the number in a note (“INV-0042 voided before send — replaced by INV-0043”).
If you reissue a corrected invoice, issue a credit note for the original and a new invoice with a new number. Never edit and resend the same number.
Keep your numbering records alongside your invoices for the ATO’s 5-year minimum retention period. See how long to keep invoices for the rules.

Why auto-numbering matters

The two ways a small business breaks its own numbering:

Manual typing. You type INV-0043 when the last one was INV-0042. Then next week you forget and type INV-0043 again. Now there are two INV-0043s and a missing INV-0044. Your bookkeeper will hate you.
Multiple devices, no source of truth. You issue an invoice from a Word doc on your laptop, then another from your phone using a different template. Both think they’re INV-0042.

Auto-numbering inside an invoicing tool is the cure. Every new invoice gets the next number atomically, no matter which device you’re on. If you cancel a draft, the number is either skipped or recycled depending on your policy — but it’s never duplicated.

How Free Invoice App handles invoice numbers

Free Invoice App auto-increments your invoice numbers from your starting value. The default format is INV-0001 with leading zeros and you can change the prefix to anything (your initials, your business code) in settings. Cancellations are tracked with a note so gaps are explained. Credit notes use a separate CN- prefix so they’re obviously distinct in the audit trail. Get started free — your numbering stays consistent from invoice #1.

Frequently asked questions

Does the ATO require a specific format?

No. Only that each number is unique. Sequential formats like INV-0001 are simply the easiest to audit.

Can I start at something other than 1?

Yes. Many start at 1001 or 10001 to avoid looking obviously brand new. The only rule is uniqueness.

What happens if I skip a number?

Nothing legal. But document why — e.g. “INV-0042 voided before send” — so audit gaps have a clear explanation.

Can I reuse a number?

Never. Reuse creates ambiguity, breaks reconciliation, and looks suspicious in an audit. If replacing a cancelled invoice, issue a credit note and use a new number.

Should I include the year?

Optional. Useful for high-volume businesses; overkill for sole traders sending a few invoices a month.