GST is one of those things every Australian small business owner has heard of, and most can technically define, but far fewer feel genuinely comfortable with day to day. That gap between knowing the term and understanding the mechanics is where most GST mistakes happen. This guide is a plain-English walk-through of how GST actually works, without the jargon.
What GST actually is
GST — Goods and Services Tax — is a 10% tax added to most goods, services and other things sold or consumed in Australia. If your business is registered for GST, you generally add 10% to your prices for taxable sales, collect that amount from your customers, and pass it on to the ATO. In return, you can generally claim back the GST included in what you buy for the business.
The important mental model here: GST isn't really your money, either coming in or going out. When you collect it from a customer, you're collecting it on the ATO's behalf. When you pay it on a purchase, you can usually claim it back. Businesses that treat GST as part of their own revenue or their own cost tend to get their numbers — and their cash flow expectations — wrong.
Do you need to register?
Most businesses need to register for GST once their turnover reaches the ATO's registration threshold. Some categories of business — like taxi and rideshare drivers — need to register regardless of turnover. Some businesses register voluntarily even below the threshold, often because it lets them claim GST credits on setup costs.
The exact threshold changes over time and depends on your circumstances, so it's worth checking current figures on ato.gov.au or with a registered tax agent rather than relying on a number you heard a while ago.
The 1/11th rule
Once you're registered, the core piece of arithmetic you'll use constantly is this: for a GST-inclusive price, the GST component is 1/11th of the total.
Take a $220 GST-inclusive invoice. The GST is $220 ÷ 11 = $20. The value of the sale before GST is $200. This works because a $200 sale plus 10% GST ($20) equals $220 — so working backwards from the total, GST is always one-eleventh of it.
This one calculation underpins almost everything else: what you report as GST collected, what you can claim as GST paid, and what shows up on your BAS.
GST collected vs GST paid
Every reporting period, there are two sides to your GST position.
GST collected (often labelled 1A) is the GST you've charged on your sales. GST paid (1B) is the GST included in what you've bought for the business — assuming those purchases are eligible for a credit.
The difference between the two is roughly what you owe the ATO, or what they owe you, for that period. If you collected more GST than you paid, you generally remit the difference. If you paid more than you collected — which can happen in a period with a large equipment purchase, for example — you may be entitled to a refund.
What you can (and can't) claim GST credits on
Not every purchase gives you a GST credit, even if GST was charged on it. Broadly, a purchase needs to be:
- For a creditable purpose (used in running your business, not for private use)
- From a supplier registered for GST
- Supported by a valid tax invoice, for purchases over the ATO's invoice threshold
Some expense categories have specific restrictions — certain entertainment expenses, for instance, often aren't fully claimable even though GST was charged. If a purchase is used partly for business and partly privately, only the business-use portion is generally claimable.
This is genuinely one of the areas where small businesses lose money without realising it — either by not claiming credits they're entitled to, or by claiming ones they're not. It's worth having a registered tax agent sanity-check your approach, particularly for larger or unusual purchases.
Common GST mistakes
Charging GST without being registered. If you're not registered for GST, your prices shouldn't include a GST component — and you can't claim GST credits on your own purchases either.
Treating GST collected as revenue. It's tempting to look at a big invoice and feel like all of it is yours. A tenth of it (roughly) is money you're holding for the ATO, not business income.
Not keeping tax invoices. Without a valid tax invoice for purchases over the threshold, you generally can't claim the GST credit — even if you definitely paid it.
Guessing instead of calculating. GST on each transaction should be calculated precisely, not estimated. Small rounding habits compound into real discrepancies by the time a BAS period ends.
Making GST a non-event
The businesses that find GST stressful are usually the ones dealing with it in a single stressful block once a quarter — reconstructing months of sales and purchases, trying to remember which invoices included GST, and hoping the numbers reconcile. The businesses that find it straightforward are the ones where GST is calculated automatically on every transaction as it's recorded, so the quarterly total is just a sum of numbers that were already correct.
GST itself isn't complicated arithmetic. What makes it feel hard is doing that arithmetic retroactively, at scale, under a deadline. Fix the timing, and GST stops being a source of dread.
GST and cash flow
One of the most underrated effects of GST is on cash flow, particularly for businesses with irregular income. Because the GST portion of every sale isn't really yours, it's easy to accidentally spend it — especially if you're not setting it aside as it comes in. A business that collects $5,000 in GST across a quarter but hasn't put any of it aside can find itself short when the BAS is due, even if the business itself is genuinely profitable.
A simple habit that helps: treat the GST component of each sale as already spoken for, the moment it lands. Some businesses do this literally, transferring the GST portion of each payment into a separate account as it arrives, so the BAS due date never requires finding money that already feels spent. You don't need a formal system to benefit from the underlying idea — just don't count GST collected as available cash.
What changes once you're registered
Registering for GST isn't just an administrative formality — it changes how you price things, invoice, and report. Your invoices need to clearly show the GST component for GST-registered customers (who will often want to claim it back themselves). Your pricing conversations may need to specify whether a quoted figure is GST-inclusive or exclusive, since the two can differ materially on a large invoice. And your reporting obligations move from "none" to "regular," typically quarterly.
None of this is difficult once you're used to it, but it is a step change from pre-registration, which is part of why the decision to register — voluntarily, before you're required to — is worth thinking through rather than defaulting into.
Where to check current rules
GST registration thresholds, invoice thresholds, and specific category rules do change over time, and getting a detail wrong can be costly — either in credits you didn't claim, or ones you claimed incorrectly. The ATO's own website is the authoritative source for current figures, and a registered tax agent or BAS agent can confirm how the rules apply to your specific business. Treat this guide as the mental model for how GST works, and treat ato.gov.au or your agent as the source for the exact numbers that apply right now.
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