ABN Validation and TFN Check for a New Client

Check an ABN against ABN Lookup, then match the business details to your client. Check a TFN’s length and number rule inside secure software. A number that passes a calculation can still belong to someone else.

ABN vs TFN: What Each Number is

An Australian Business Number (ABN) identifies a business on the public Australian Business Register (ABR). A Tax File Number (TFN) identifies a taxpayer to the Australian Taxation Office (ATO). TFNs are private and their collection and use are restricted by law.

Number	Length	What You Check
ABN	11 digits	Number rule, public registration, entity name and status
TFN	Usually 9 digits, with older 8-digit numbers also recognised	Length and number rule, followed by taxpayer matching through an authorised ATO service
Australian Company Number (ACN)	9 digits	Number rule and ASIC company register details

The Australian TFN description supplied to the OECD recognises both TFN lengths. Do not reject a client’s eight-digit number solely because it has fewer than nine digits. The receiving ATO service’s input rules also matter.

A sole trader uses their individual TFN for their tax affairs and an ABN for their business dealings. A company has its own identifiers. Keep each person’s TFN separate from the company’s TFN and from a trust or partnership’s TFN. Use the entity TFN guide when you need to establish which entity requires one.

A company’s ABN usually contains its ACN as the last nine digits. Compare those digits with the ACN on the company register, then match the company name. ASIC’s ACN guidance explains this relationship. A similar trading name does not establish a match. The guide to checking ASIC company details explains the company details to check.

How to Check an ABN Number on ABN Lookup

Use ABN Lookup to confirm the public registration for the exact entity your practice will act for. You do not need a web-services account for a website search.

1. Enter the ABN in the search box. If the client has not supplied it, enter their entity name, registered business name or ACN.
2. Open the matching result. A name search can return several entities, so compare the state and postcode as well as the name.
3. Read the entity name and entity type. Check that an individual/sole trader, company, trust or partnership matches the form.
4. Read the ABN status and its effective date. Check GST registration separately, including when it starts or ends.
5. Compare the registered business names and main business location with the client’s answers. Keep a record of the search and any mismatch you resolve.

ABN Lookup’s search instructions explain the active and all-ABN result tabs. Use the all-ABN tab when an old name or cancelled registration might explain the result.

For example, a fictional client selects company on the form but supplies an ABN whose entity type is individual/sole trader. Ask which entity owns the business and which entity needs your service. Obtain the correct number before using it for the company’s work.

How to Check If an ABN is Active

Read the ABN status on the current details page. The active or cancelled label applies from the effective date beside it. An active ABN does not establish GST registration.

Open Historical details when you need the position for an earlier period. The ABN status rows show active and cancelled periods with From and To fields. GST rows have their own periods. A later active period after a cancelled period shows when registration resumed.

Match the period you are working on to those rows. A client’s ABN can be active now while their GST registration ended earlier. Keep the ABN and GST findings separate in your file.

If the ABN is cancelled, confirm whether the client stopped trading, changed entity or resumed business. Check their entitlement before treating reactivation as the answer. The ABN application and reactivation guide covers that next step. Resolve any registration mismatch before lodging information that relies on it.

Checking Whether a Client Already Has an ABN

Search a sole trader’s legal name and any business name they have used. Refine a broad result with state or postcode, then open the candidate records. Include cancelled records before concluding that no ABN exists.

Public results can contain limited details or withheld information. A missing name-search result does not prove that the client has never held an ABN. Ask the client to confirm their business details through their own ATO online services or existing registration correspondence.

Do not replace an uncertain result with another person’s similar name. Record what the client confirms and match the number back to the correct entity.

The TFN Check: Format and Check Digit

A TFN format check tests the supplied digits against a mathematical rule. Each position has a fixed weight. Software multiplies each digit by its weight, adds the results and checks whether the total divides evenly by 11.

Use your secure tax software’s TFN check. Keep the number as text so formatting does not remove digits. Remove permitted display spaces for the calculation, without changing the number. Do not paste a client’s TFN into a public online checker.

If the test fails, ask the client to check the number against their ATO record and return it through your secure collection method. Never change a digit simply to make the calculation pass.

If the test passes, the result establishes only that the supplied digits satisfy the rule. It does not establish that the ATO issued that number to this client. A copied number from another taxpayer can pass the same test.

An authorised agent’s client-addition search uses the identifier with other matching details. For an individual, the ATO’s instructions ask for the date of birth. For a non-individual TFN search, they ask for the name or ABN. Check that the returned taxpayer matches your client before completing the authorised linking steps. A mismatch requires correction or ATO assistance, even when the local number test passes.

The ABN Calculation

The ABR’s published ABN method uses a different calculation. Subtract one from the first digit, multiply the resulting digits by the weights below, then divide their total by 89. The remainder must be zero.

Use the fictional, deliberately incorrect input 12 345 678 901 for this arithmetic example. It does not identify a business registration.

Position	Digit After Subtracting One	Weight	Product
1	0	10	0
2	2	1	2
3	3	3	9
4	4	5	20
5	5	7	35
6	6	9	54
7	7	11	77
8	8	13	104
9	9	15	135
10	0	17	0
11	1	19	19

The total is 455. Dividing by 89 gives five with a remainder of 10, so this input fails. A zero remainder would still require an ABN Lookup match.

The ACN Calculation

ASIC’s document messages specification sets out the ACN rule. Multiply the first eight digits by 8, 7, 6, 5, 4, 3, 2 and 1 respectively. Add the products and find the remainder after dividing by 10.

Subtract that remainder from 10. If the result is 10, use zero. The result must equal the ninth digit. A passing calculation still needs a company register match.

Checking a Whole Client List at Once

ABN Lookup’s free web services let software look up many ABNs in a repeatable batch. Read the user guide and agreement, complete registration and use the access identifier sent by email. The service retrieves register details. It does not update registrations.

For a smaller list, use a restricted spreadsheet containing ABNs and entity names, without TFNs. Set up a format-result column first. Add separate columns for the ABN status, GST status, entity match and the staff member handling exceptions. Look up each passing ABN and record the outcome. A failed calculation must return for correction before the register check.

For automated checks, the web-services methods support searches by ABN and an option to include historical details. A batch needs one result or recorded error for every input. Do not interpret a failed request as a cancelled ABN.

A practice cannot use ABN Lookup or a public bulk checker to confirm TFN ownership against ATO records. Keep list-wide TFN checks inside authorised secure software. Their format results remain separate from the ATO matching performed for an authorised client relationship.

Recheck ABN and GST status before each lodgment season as a practice procedure. Check sooner when a client reports a business change or when you need the registration position for a specific period.

Catching Number Errors at Onboarding

Check numbers while the client completes the form, then resolve register mismatches before staff use the answers. An immediate error message lets the client correct an omitted or swapped digit while their source record is nearby.

Collect individual TFNs only for an authorised purpose. The Privacy (Tax File Number) Rule 2015 governs individual TFN information. Other tax-law protections also apply to entity TFNs.

Before collection, identify the authorising law and purpose. Explain that refusing to quote a TFN is not an offence and explain the consequences of refusal. Limit access to staff who need it for the authorised work.

Protect stored numbers and keep them out of ordinary correspondence. Set retention according to the applicable legal requirement and authorised need, then securely destroy or permanently de-identify information when neither a legal retention requirement nor an authorised need remains.

Tax Genie checks ABN, ACN and TFN format as the client types and matches ABNs against the register. After the form arrives, number and register checks run before AI review. Tax Genie removes every TFN before AI runs. Fixable issues return to the client, with only the affected sections reopened.

Tax Genie keeps TFNs hidden until signed-in staff reveal them and logs every reveal. TFNs stay out of emails, exports and generated documents. Staff corrections record who changed what and when. The AI client onboarding guide explains how those checks fit into the request.

Before relying on the completed form, confirm that each number belongs to the right person or entity and resolve the relevant register or ATO mismatch. A format pass or AI review does not establish identity or ownership of a number.