ATO Car Expense Substantiation Rules: What You Must Prove

The ATO has strict rules about substantiation — the evidence you need to prove your tax deduction claims are legitimate. For car expenses, these rules determine whether your deduction stands or falls if the ATO comes knocking. This guide explains the substantiation framework, the exceptions that apply, and exactly what evidence you should keep.

What is substantiation?

Substantiation simply means having documentary evidence to support your claims. Under Australian tax law (specifically Division 900 of the Income Tax Assessment Act 1997), you must be able to substantiate most work-related expense claims with written evidence — receipts, invoices, bank statements, or other documents.

If you cannot substantiate a claim, the ATO can disallow it entirely. This applies to car expenses just as much as any other deduction.

The substantiation exception for car expenses

Here is where it gets interesting. The ATO provides a substantiation exception for car expense claims made under the cents per kilometre method. This means:

• You do not need receipts for fuel, insurance, registration, or other running costs
• You do not need a formal 12-week logbook
• You do need to show how you calculated your business kilometres — a reasonable basis for the claim

This exception makes the cents per km method attractive for taxpayers who want a simple deduction without maintaining a filing cabinet of receipts.

However, the exception has limits. You can only claim up to 5,000 business km per car per year under this method, for a maximum deduction of $4,400 (at the 2025–26 rate of 88 cents per km).

What “reasonable basis” means

Even under the substantiation exception, the ATO expects you to be able to explain how you arrived at your kilometre figure. Acceptable approaches include:

• A diary of business trips showing dates, destinations, and estimated distances
• A digital trip log from a mileage-tracking app
• A calculation based on a regular pattern (e.g. “I visit clients twice a week, 35 km round trip, for 48 working weeks = 3,360 km”)

The ATO does not accept a bare number with no supporting explanation.

Full substantiation: the logbook method

If you choose the logbook method, the substantiation exception does not apply. You must keep:

1. A valid 12-week logbook

Your logbook must cover a continuous 12-week period and record every trip (business and private). Each entry requires:

• Date
• Odometer reading at start and end
• Kilometres driven
• Purpose of the trip (business or private)
• Destination details

The logbook establishes your business-use percentage, which you apply to your total running costs for the year.

2. Odometer readings

Record your odometer at:

• The start and end of the 12-week logbook period
• 1 July and 30 June of each income year you rely on the logbook

3. Written evidence for all running costs

You need receipts, invoices, or statements for every expense you claim:

• Fuel (receipts or credit card/fuel card statements)
• Registration (renewal notices)
• Insurance (policy documents or payment receipts)
• Servicing and repairs (mechanic invoices)
• Tyres
• Car loan interest (loan statements)
• Depreciation (purchase price documentation)

Each piece of written evidence must show the date, amount, nature of the expense, and the supplier’s name.

For a deeper guide on all record-keeping requirements, see our ATO motor vehicle record-keeping guide.

The $300 substantiation threshold

There is a common misconception about a “$300 rule” for work-related expenses. Here is how it actually works:

• If your total work-related expense claims (not just car expenses) are $300 or less, you do not need written evidence — but you must still be able to show how you calculated the amount.
• If your total claims exceed $300, you need written evidence for every claim — including car expenses.

In practice, most people who claim car expenses will also claim other work-related deductions (phone, uniform, tools), so the total almost always exceeds $300. Do not rely on this threshold as a shortcut.

Car expenses and the $300 threshold

An important nuance: car expenses claimed under the cents per km method are excluded from the $300 threshold calculation. The substantiation exception for cents per km operates independently. So even if your non-car work-related claims are under $300, your cents per km car claim is covered by its own separate exception.

What happens if you fail to substantiate?

If the ATO audits your return and you cannot provide adequate evidence:

• The deduction may be reduced to the amount you can substantiate
• The deduction may be disallowed entirely
• You may face interest charges on the underpaid tax
• In serious cases, the ATO may impose penalties — typically 25% to 75% of the shortfall amount, depending on whether the error was careless, reckless, or intentional

The ATO has flagged work-related car expenses as a priority area for compliance activity. For details on what triggers an audit, see our guide on ATO car expense audit red flags.

Digital substantiation

The ATO accepts digital records, including:

• Scanned or photographed receipts — as long as the key details (date, amount, supplier) are legible
• App-generated trip logs — from mileage-tracking apps that record date, distance, route, and purpose
• Electronic bank and credit card statements — as supporting evidence for expenses
• Cloud-stored documents — provided they are secure and backed up

Digital records must be a true and clear copy of the original. If you photograph a receipt, ensure the image is readable and stored securely for at least five years.

Practical tips for bulletproof substantiation

1. Use a tracking app — automated kilometre logs are more reliable and consistent than manual diaries.
2. Photograph receipts immediately — thermal receipts fade within months. Snap a photo on the day of purchase.
3. Keep a simple trip diary — even for cents per km claims, a dated list of trips with destinations and purposes is strong evidence.
4. Store everything for five years — from the date you lodge your return, not from the date of the expense.
5. Separate your records by year — label folders clearly so you can find everything quickly if the ATO writes to you.
6. Review your logbook — if your circumstances change significantly, start a new 12-week logbook rather than relying on an outdated one.

Tripbook handles substantiation for you

Keeping up with substantiation requirements is the part of tax that nobody enjoys. Tripbook takes care of the hard part by automatically tracking every trip, recording the date, distance, and route, and letting you tag each trip as business or personal. Your kilometre records are always ATO-ready — whether you use cents per km or the logbook method.