GST on Mixed-Use Purchases: What You Can (and Can't) Claim

Why mixed-use purchases trip people up

Plenty of things a sole trader or small business buys aren't used exclusively for the business. A mobile phone plan might carry both client calls and personal messages. A car might do the school run on Monday and a client visit on Tuesday. A home internet connection might run invoicing software in the morning and streaming in the evening.

These mixed-use purchases sit in an awkward middle ground. They're not fully business, so you can't treat the whole cost as a business expense. But they're not fully personal either, so ignoring the business portion leaves money on the table. Getting this apportionment right matters for both your income tax deductions and, separately, for any GST credits you claim.

This post looks specifically at how GST interacts with mixed-use purchases — a slightly different question from "can I deduct this," and one that catches out even business owners who are otherwise careful with their records.

GST credits are about the business-use portion, not the whole purchase

If you're registered for GST, you're generally entitled to claim a GST credit (sometimes called an input tax credit) on purchases you use to make taxable supplies in your business — but only to the extent the purchase is actually used for the business.

The mechanics are the same 1/11th calculation used everywhere else in the GST system: if a GST-inclusive purchase is used 100% for business, the GST component is 1/11th of the price paid, and that's the credit you claim. If the purchase is mixed-use, the same 1/11th calculation applies to the price, but then you apply a business-use percentage on top of it.

For example, if you buy something for a GST-inclusive price and estimate the item is used 60% for business and 40% personally, the GST credit you claim is generally 60% of the 1/11th GST component — not the full 1/11th. Claiming the full amount when the purchase isn't fully business-related is one of the more common overclaims the ATO looks for, and it's usually not deliberate. It happens because the invoice shows one GST figure and it's easy to assume the whole thing is claimable.

Working out a reasonable business-use percentage

There's no single formula that fits every kind of mixed-use expense, because the fairest way to apportion a phone bill is different from the fairest way to apportion a vehicle. What matters is that the percentage is reasonable, consistent, and — ideally — backed by some kind of record rather than a round number picked because it looks tidy.

Some common approaches:

Usage logs or diaries. For a vehicle, a logbook kept over a representative period (commonly 12 weeks) showing business versus personal kilometres is the most defensible method. For a phone, some people keep a call and data log for a few weeks each year to establish a ratio they then apply consistently.

Time-based estimates. For something like home internet or a home office space, business-use percentage is often estimated based on hours used for work versus other purposes, or the proportion of the home floor area used for business.

Itemised billing. Some phone and software plans let you separate business and personal use directly through itemised statements, which removes the guesswork entirely.

Whatever method you use, the important habit is applying it consistently and keeping a note of how you arrived at the percentage. If your business-use pattern changes significantly — say, you start using your car for far more client work — it's reasonable to revisit and update the percentage rather than leaving an old estimate in place indefinitely.

Capital purchases follow the same logic, at a larger scale

Mixed-use apportionment isn't limited to small recurring costs like phone bills. It applies just as much to bigger purchases — a laptop, a vehicle, office equipment — where the GST credit and the ongoing depreciation deductions both need to reflect the actual business-use proportion, not the sticker price.

Because the dollar amounts involved are larger, it's worth being a little more careful with these. A reasonable, well-documented estimate for a $60 monthly phone bill is unlikely to attract much scrutiny either way. A poorly justified 90% business-use claim on a new vehicle is a different matter. This is one of the areas where it's genuinely worth checking your approach with a registered tax agent or bookkeeper, particularly for larger purchases, since the right treatment can depend on details specific to your situation.

Recording mixed-use purchases as you go

The cleanest way to handle mixed-use expenses is to record them as mixed-use at the point of entry, rather than trying to remember which purchases needed splitting when it's time to lodge a BAS or prepare your tax return months later.

In practice, this generally means:

• Capturing the full amount and GST from the invoice or receipt as normal.
• Recording the business-use percentage you're applying, and a short note on how you arrived at it.
• Letting your GST credit and expense claim reflect only the business-use portion.

Tracking this at the transaction level, rather than trying to apply a blanket adjustment at BAS time, tends to produce a more accurate — and more defensible — set of records. It also means you're not relying on memory to reconstruct why a percentage was chosen six or twelve months after the purchase.

Why this connects to broader record-keeping habits

Mixed-use purchases are a good example of why GST record-keeping benefits from being handled continuously rather than in a rush before each BAS is due. A single missed apportionment on a recurring expense, if it goes unnoticed, tends to repeat itself every time that expense appears again. Catching it early — ideally the first time the purchase is recorded — saves having to go back and correct a pattern of entries later.

It's also a reminder that GST and income tax deductions, while related, aren't identical questions. A purchase might be deductible for income tax purposes at one percentage and eligible for a GST credit at a different, though usually matching, percentage, depending on how the purchase is used and structured. Keeping clear notes on business-use percentages as you go makes it far easier to answer both questions consistently, rather than reconstructing the reasoning from scratch at tax time.

The general takeaway

Mixed-use purchases are a normal part of running a small business, and GST doesn't require you to avoid them — it just asks that the credit you claim reflects genuine business use rather than the full purchase price. A consistent, reasonably documented method for estimating business-use percentage, applied at the time each transaction is recorded, is generally enough to keep this manageable. For anything with real dollar value attached — vehicles, equipment, larger recurring contracts — it's worth checking your specific approach against ato.gov.au or with a registered tax agent, since the right treatment can vary based on how the asset or expense is actually used in your business.