What Is an Invoice Number?

Across Australia, invoices move quietly through inboxes every day. A technician finishes a repair, a designer hands over a final draft, an online business completes another sale. Each invoice looks simple enough, until you notice the small code printed at the top. The invoice number.

You may not pay much attention to the invoice number at first, but it plays an important part in your financial process. It identifies each invoice, keeps payments organised, and supports accurate bookkeeping. Without it, managing invoices becomes far more complex.

This guide covers what an invoice number is, why businesses rely on it, and how to build a numbering system that stays simple, consistent, and audit-ready.

Table of contents
What is an invoice number 4 different ways to assign invoice numbers Best practices Wise Business: Simplify global invoicing and reconciliation FAQs

What is an invoice number

An invoice number is a unique code that identifies a specific invoice. It separates one invoice from another and helps both the business and the customer keep track of what the invoice refers to.

You can use numbers, letters, dates, or a mix of all three, as long as each invoice carries a number that you never repeat.

Invoice number tells you

• Which job the invoice refers to
• When the invoice went out
• Who the invoice belongs to
• How to match a payment to the correct document

Examples:

• INV-0034
• 20240201-001
• AUS-0145
• Client12-009

These formats differ, but they all meet one requirement: Unique Identification

Why are invoice numbers important?

• They support faster payments: Customers match their payments to the invoice number. A clear code avoids delays and back-and-forth emails like “Which invoice does this payment relate to?”
• They build reliable accounting records: Invoice numbers help create a complete financial trail. Australian businesses need clean books for: BAS statements, GST reconciliation, EOFY reporting, and cash-flow management.
• They reduce errors: No business wants duplicate payments, missing revenue, or misapplied transactions. A good numbering system closes the door on these mistakes.
• They strengthen audit compliance: The ATO expects accurate documentation. A tidy, sequential invoice history shows responsible recordkeeping and lowers audit risk.
• They give customers clarity: Most clients, especially larger Australian organisations, cannot pay without an invoice number. Some accounting systems reject invoices that do not include one.

Difference between an invoice number and a receipt number

An invoice number shows a request for payment. It tells the customer what they owe and helps you track the amount until it is paid. A receipt number confirms the payment. It shows that the customer has paid and acts as proof of the completed transaction.

Invoice Number	Receipt Number
Identifies money owed	Identifies money received
Appears before payment	Appears after payment
Used to track outstanding invoices	Used to confirm completed payments
Helps organise billing and follow-ups	Acts as official proof of payment
Part of the invoicing process	Part of the payment/recordkeeping process

Example:

1. You send a client an invoice for 850 AUD.
2. The invoice carries the number INV-0241.
3. A week later, the client pays the 850 AUD.
4. You issue a receipt with the number RCT-0241.
5. The invoice number shows what the client owes,
   and the receipt number shows what the client paid.

4 different ways to assign invoice numbers

Every business runs differently, so the best invoice number format depends on your workflow. Some common methods that Australian businesses use for invoicing across trades, services, retail, and online operations are below.

1. Sequential numbering

The simplest and most universal method.

Examples: 001, 002, 003

This is more suitable for sole traders, freelancers, and businesses dealing with a low to moderate quantity of invoices. It keeps everything straightforward and easy to follow.

Each invoice follows the next in clear order, so you always know which one came first. This way, customers can also refer to and pay the correct invoice more easily. Accountants like this method because it creates a neat, organised record with no gaps or confusion.

2. Chronological numbering

This format includes the date in the invoice number.

Examples:

• 020324-001 → 2 March 2024
• 140124-002 → 14 January 2024

This method applies to companies that issue a large number of invoices in one day and to industries where time is essential, such as logistics, compliance, or building trades.

It makes recordkeeping more transparent and efficient, as the date is combined with the invoice number. You can tell at a glance when it went out without opening the file or checking your system. This helps teams organise documents by day and keep track of workload without mixing up invoices from different periods. For fast-moving businesses, everything stays organised and traceable.

3. Customer-based numbering

Each client gets a unique code.

Example:

• Client ID: CN004
• Invoices: CN004-001, CN004-002

It works well for agencies, consulting firms, and service businesses with long-term clients. Each customer gets a separate numbering sequence that keeps their invoices neatly grouped. When preparing statements or reviewing past work, you will be able to find all the invoices for a particular customer in one spot.

It also makes account reconciliation easier, as clients frequently send multiple payments across the year. Overall, it provides clarity of structure for businesses involved in ongoing relationships.

4. Project-based numbering

Used when billing happens across large projects.

Example:
Project Code: PRJ27 → Invoices: PRJ27-001, PRJ27-002

This method is ideal for construction firms, IT development teams, and engineering and design firms, as well as marketing agencies that handle big, long-term projects. It sets each project up with its own unique invoice sequence, segregating revenue from the rest of the business for that project alone.

This makes the review of progress easier, along with the billing history, to measure profitability per project. It also supports clearer reporting, especially when multiple projects run at the same time.

Best practices when creating an invoice number

A well-designed numbering system does not have to be complex. It must only stay consistent, scalable, and easy to follow.

1. Select a format that can grow
Use a numbering style that can handle higher volumes later on. A four-digit sequence like 0001 keeps the format stable as your invoice count rises and avoids issues when the numbers start to scale.

2. Use prefixes for structure
Prefixes help sort invoices by type, year, or category. Codes such as AUS-0001, SERV-0100, or 2024-INV-015 give quick context and make your records easier to navigate as the list gets longer.

3. Keep the format consistent
Stick to one format across all invoices. When the pattern stays the same, your team, customers, and accountant can reconcile payments and track records without second-guessing anything.

4. Never repeat an invoice number
Give each invoice its own unique number, even if you cancel or void it later. A clear, uninterrupted sequence maintains transparency and reduces the risk of duplicate or mismatched entries.

5. Avoid overly complex codes
Overly long or complicated invoice numbers slow down processing and increase mistakes. Use simple formats like INV-0256 and avoid confusing codes like XJ72-PL8-9293-ALPHA-INV-004 or long descriptive strings. A clean number keeps invoicing fast and easy for everyone.

6. Store invoices digitally
Digital copies are easier to search, protect, and share, especially during tax reviews or BAS lodgements. PDF invoices provide a clear, consistent record that stays intact over time. Digital storage keeps your financial history tidy and accessible.

7. Use software that assigns numbers automatically
Automation removes the risk of typos, skipped numbers, or duplicates. Software creates a clean sequence and links invoices to payments, making reconciliation much smoother. It also saves time and reduces manual admin work.

Wise Business: Simplify global invoicing and reconciliation

Wise Business helps streamline this global workflow by providing an solution tailored for international clients. You can issue invoices that include your own local account details for major currencies, allowing customers to pay in their local currency without high transfer fees or confusing exchange rate markups.

This makes it easier to match incoming funds to the correct invoice number, ensuring your global bookkeeping remains just as tidy as your local records.

Expanding a business globally opens up exciting opportunities, but also new challenges like receiving payments across borders. Hidden foreign transaction fees and hefty currency conversions involved with international payments can eat into your profits and time.

Wise Business serves as a cost-effective solution where you can receive money from around the world at the speed and price of local payments.

Transform the way you receive payments with Wise Business:

• One-time fee of 65 AUD for local account details in 8+ currencies, including AUD, NZD, USD, and more—no recurring fees
• One account to hold, send, and convert money with no hidden fees or exchange rate markups
• Create and send professional invoices directly to your customers through Wise Business
• Create payment links to request money in specific currencies
• Seamlessly receive payments from customers, online sales, or PSPs like Stripe and Amazon.
• Wise is safe and secure - Trusted by 13 million people and counting

Sign up for the Wise Business account! 🚀

This general advice does not take into account your objectives, financial circumstances or needs and you should consider if it is appropriate for you.

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FAQs

1. Where do I place the invoice number on a document?
Place it at the top of the invoice, usually near the invoice date, invoice title, or seller details. This makes it easy to identify.

2. What happens if I accidentally skip an invoice number?
You do not need to fix the gap. Just continue with the next number. The skipped number stays unused but does not break compliance.

3. How long should an invoice number be?
There is no fixed length. Most businesses use 4–10 characters to ensure enough room for growth.

4. Can I use letters in my invoice number?
Yes. You can use letters for prefixes, client codes, or project codes. They help with organisation and tracking.

5. Do proforma invoices need a unique invoice number?
No. A proforma invoice is a quote, not a record for payment. You can label it “Quote” or “Proforma” without assigning an invoice number.