Australian TFN: format, validation and detection

What it is

The TFN identifies taxpayers in Australia. Maskflare's Australia pack covers it as a tax ID, with an unformatted sibling for runs with no spaces.

In Maskflare this is the detector au_tfn, labelled "Australia - Tax file number (TFN)", category tax id, in the Australia pack.

Format and layout

• Nine digits grouped 3-3-3 with spaces, for example 123 456 782, or an eight-digit form (3-3-2).
• All digits cannot be the same.

How the check works

Multiply each digit by its weight and add the products. The total must be a multiple of 11.

Worked example

1. Digits 1 2 3 4 5 6 7 8 2 with weights 1 4 3 7 5 8 6 9 10 give 1, 8, 9, 28, 25, 48, 42, 72, 20.
2. The total is 253, which is 23 times 11, so the number passes.

Valid and invalid examples

In running text the valid example reads, for instance, TFN 123 456 782. The invalid one is left alone: a value that fails the validator is treated as an ordinary string.

Regex and validation caveats

• About one in 11 random nine-digit numbers passes, so the unformatted sibling detector needs a keyword before the number; the spaced form is matched on its own.
• Australia has no compliance profile in Maskflare, so the detector depends on the primary country setting.

How to detect it in LLM traffic

Australian TFN values reach an LLM inside ordinary work rather than as standalone data: a pasted block of HR or payroll rows, a customer-support thread, an onboarding or identity-check form, a spreadsheet someone wants summarised, or an email they want rewritten. The model needs none of the digits for any of those jobs, so the safest place to remove them is before the request leaves the company.

Maskflare reads the whole request body, including the system prompt, earlier messages and tool arguments, so a number buried in the fourth message of a long chat is found as readily as one in the latest message. Each match is replaced with a token such as mf_mask_d7e0 before the request goes out, and with response restoration turned on the real value is put back into the answer for the person who is allowed to see it.

If you build the detection yourself

1. Strip spaces, dots and hyphens before running any arithmetic, but remember where they were so the original text can be replaced exactly.
2. Match the shape with a pattern anchored on word boundaries, so that a long digit string is not read as several shorter Australian TFN candidates.
3. Run the check from the section above. A failing check is almost always a typo or a made-up value; a passing one removes most look-alikes.
4. Replace the value before the text is sent anywhere, and keep a mapping if the answer has to be restored afterwards.

How Maskflare handles it

• Protect now, Finance - Protect now, Healthcare and Developer: mask (a reversible token) when Australia is a primary country.
• Monitor first: alert (counted, forwarded unchanged) when Australia is a primary country.

The available actions are mask (reversible token), redact, partial, hash, format-preserving encryption, generalize, alert and block. Masked values are kept encrypted for 30 days so that answers can be restored, then they expire; prompts and responses are not logged. The full list of detectors is in the detector reference.

Compliance profiles that include it

None of the 6 compliance profiles (HIPAA Safe Harbor, GDPR (EU/EEA), UK GDPR, PCI DSS v4.0, US PII (CCPA/CPRA) and Swiss nDSG health data) includes the Australia pack, so no profile turns this detector on. It runs when the pack is enabled directly, or when Australia is chosen as a primary country.