A plain-English reference guide for Australian business owners
At some point, almost every new business owner ends up staring at a form asking for a number they don't immediately recognise — and quietly wondering whether it's the same number they gave someone else last week.
ABN. ACN. TFN. GST. Four sets of letters, four different jobs.
This guide is meant to be a reference you come back to, not something you need to memorise in one sitting.
The short answer
An ABN identifies your business. An ACN identifies a registered company. A TFN identifies you — or your entity — for income tax. GST is a tax, not a number you're issued; it's something you register for once you meet certain conditions.
They're issued by different bodies, for different purposes, and you can hold more than one at the same time.
At a glance
| Term | What it is | Issued by | Who has one |
|---|---|---|---|
| ABN | 11-digit business identifier | Australian Business Register (ATO) | Sole traders, partnerships, companies, trusts carrying on an enterprise |
| ACN | 9-digit company identifier | ASIC | Registered companies only |
| TFN | Personal or entity tax file number | ATO | Every individual, and separately, companies and trusts |
| GST | A 10% tax on most goods and services | Registered with the ATO | Businesses that are registered — compulsory above the turnover threshold |
ABN — Australian Business Number
An 11-digit number that identifies your business to the ATO, and to the businesses and government agencies you deal with. It's free to apply for, and you'll use it on invoices, when registering for GST, and whenever you're dealing with suppliers or clients.
Without one, clients may be required to withhold tax at the top rate from payments to you, and you can't register for GST.
ACN — Australian Company Number
A 9-digit number issued by ASIC the moment a company is registered. Only companies get an ACN — sole traders, partnerships and trusts don't have one, because they aren't registered with ASIC in the same way.
If a company also has an ABN, that ABN is generally the ACN with two extra digits added to the front — which is why the two numbers often look related.
TFN — Tax File Number
A TFN belongs to you as an individual and is used for lodging your personal tax return. Companies and trusts are separately issued their own TFN, distinct from any individual's.
Sole traders generally use their own personal TFN for the business. Other structures — companies, trusts, partnerships — need a TFN of their own.
The key distinction to hold onto: an ABN is public — it appears on invoices and business documents. A TFN is private and should never be handed out the way an ABN is.
GST — Goods and Services Tax
GST isn't a number you're issued — it's a tax you register for. Once registered, you add 10% GST to most sales, issue proper tax invoices, lodge Business Activity Statements, and can claim back GST included in your own business purchases.
Registration is compulsory once your GST turnover reaches $75,000 in a 12-month period ($150,000 for non-profits), and optional below that. This is covered in full in its own guide.
How they relate to each other
A sole trader typically has: an ABN, and uses their personal TFN.
A company typically has: an ACN (from ASIC), an ABN (usually the ACN plus a two-digit prefix), and its own separate TFN.
GST registration sits on top of any of these structures — it's linked to your ABN once you register.
Common points of confusion
“I have an ABN, so I must be registered for GST.” Not necessarily — an ABN and GST registration are separate. You can hold an ABN without being GST-registered.
“My company's TFN and ABN are the same thing.” They're not — they're issued for different purposes, even though the ABN may visually resemble the ACN.
“I should give out my TFN like my ABN.” Your ABN is meant to be shared. Your TFN is private and should only be given to entities with a legitimate right to it — like your employer, bank or the ATO.
When should you get professional help?
Most of this is genuinely self-service through government portals. Worth a conversation with an accountant if:
you're unsure which numbers your specific structure actually needs;
you're setting up a company and want your ABN, ACN and TFN registrations sequenced correctly; or
you suspect a number may have been used incorrectly on past invoices or tax documents.
Continue learning
→ Sole Trader vs Company — Which Business Structure Is Right for Me?
→ I've Decided to Start a Business — What Do I Actually Need to Set Up?
→ Do I Need to Register for GST?
SKC Accounting | Knowledge Centre | Knowledge Guide 003
Last reviewed: August 2026
General information only. This guide does not take into account your individual circumstances and is not a substitute for professional accounting, taxation, legal or financial advice.