How GST Works for Small Businesses in Australia: A Plain English Guide

Introduction: The $75,000 Threshold

In Australia, businesses must register for GST once annual turnover reaches $75,000 ($150,000 for non-profits). The threshold has been $75,000 since GST began on 1 July 2000 and has never changed. Taxi, limousine, and ride-sourcing drivers (Uber, Didi, Ola) must register from their very first dollar, regardless of turnover.

The GST threshold is one of the most important numbers for any Australian small business owner to understand. Crossing this threshold triggers significant changes to your business operations, including the requirement to charge GST on your sales, issue tax invoices, and lodge regular BAS statements. Many businesses, particularly in their early stages, find themselves approaching this threshold without realizing it, which can lead to compliance issues and unexpected tax liabilities.

How GST Turnover Is Actually Calculated

GST turnover is your gross business income (not profit), and the ATO uses two measures—you must register if either reaches the threshold: Current turnover (the current month plus the previous 11 months, backward-looking) and Projected turnover (the current month plus the next 11 months, forward-looking).

This means signing one large contract can trigger registration even if your historical earnings never crossed $75,000. You have 21 days from when you know (or should reasonably know) you will exceed the threshold. The ATO expects businesses to monitor their turnover actively and register as soon as they become aware that they will exceed the threshold, not after the fact.

For businesses with seasonal income, such as tourism operators or retail businesses with strong Christmas sales, the projected turnover test is particularly important. A business might have a quiet first half of the year followed by a boom in the second half, and the projected turnover test ensures that such businesses do not delay registration until it is too late.

What Counts—and What Doesn't

GST turnover includes taxable and GST-free sales (most exports, fresh food). It excludes input-taxed supplies—most notably residential rent and most financial supplies like interest and dividends. This matters for property investors who also run a business: rental income alone will not push you over the threshold.

Understanding what counts toward your GST turnover is essential for accurate compliance. Many business owners mistakenly include GST-exclusive or input-taxed supplies in their turnover calculations, leading to confusion about whether they need to register. The ATO provides detailed guidance on what constitutes GST turnover, and seeking professional advice can help you navigate these nuances.

Real Example: Sarah the Freelance Graphic Designer

Sarah, a freelance graphic designer, earned $68,000 in FY2024-25—under the threshold. In September 2025 she wins a $2,000/month retainer client. Her projected 12-month turnover is now $92,000. She must register within 21 days of September 2025, even though her trailing 12-month total is still under $75,000.

Sarah's situation is common among freelancers and service-based businesses. A single new client can push you over the threshold, and if you are not monitoring your projected turnover, you may miss the 21-day registration window. This can result in significant penalties and interest, as well as the requirement to pay GST on sales you made before you registered.

What Changes Once You Register

Once registered, you must add 10% GST to taxable sales, issue valid tax invoices, claim GST credits on business purchases, and lodge regular BAS statements (monthly, quarterly, or annually). Your pricing strategy may need to change to account for the 10% GST, and you will need to update your contracts, proposals, and invoices to reflect your new GST status.

Claiming GST credits on business purchases is one of the benefits of registration. If you are registered for GST, you can claim back the GST you pay on business expenses, reducing your overall tax burden. This can be particularly valuable for businesses with significant GST-bearing expenses such as equipment, inventory, and professional services.

Lodging BAS statements is another new obligation. Depending on your turnover, you may need to lodge monthly, quarterly, or annually. Your BAS will report your GST collected, GST paid, and any other tax obligations such as PAYG withholding or instalments. Accurate record-keeping is essential to ensure your BAS is correct and filed on time.

Should You Register Voluntarily Below the Threshold?

Yes, this can make sense if your clients are GST-registered businesses (they can claim your GST back), you have significant GST-bearing expenses you want to recover, or you expect to cross the threshold soon. If you voluntarily register, you generally must stay registered for at least 12 months.

Voluntary registration can also enhance your business's credibility. Some larger businesses and government agencies prefer to work with GST-registered suppliers because it simplifies their own BAS reporting. Being registered for GST can therefore open doors to new opportunities and clients that might otherwise be unavailable.

However, voluntary registration also means you must charge GST on your sales, which may make your prices less competitive compared to non-registered competitors. You will also need to invest time and resources into GST compliance, including record-keeping and BAS lodgement. Weighing the pros and cons carefully is essential before making a decision.

The Cost of Missing the Window

If you miss the 21-day window, the ATO can backdate your registration (up to 4 years absent fraud), meaning you owe GST on every sale since that date—even if you never charged customers for it. On $80,000 of sales, that is roughly $7,273 that you absorb out of revenue already received.

The backdating of registration can have a devastating impact on a small business's cash flow. Not only do you have to pay GST on past sales out of your own pocket, but you may also face penalties and interest for failing to register on time. The ATO can impose a penalty of up to 25% of the GST owed, in addition to the General Interest Charge (GIC) on the outstanding amount.

To avoid this situation, it is essential to monitor your turnover regularly and seek professional advice as soon as you approach the $75,000 threshold. A registered BAS agent or accountant can help you determine when you need to register and ensure you comply with all deadlines.

Common GST Mistakes to Avoid

One of the most common GST mistakes is failing to register on time. Many business owners assume they have until the end of the financial year to register, not realizing that the 21-day window applies as soon as they know or should reasonably know they will exceed the threshold. This misunderstanding can lead to costly backdated GST liabilities.

Another common mistake is misclassifying supplies. Some businesses incorrectly treat GST-free or input-taxed supplies as taxable, resulting in overpaying GST. Others incorrectly treat taxable supplies as GST-free, resulting in underpaying GST and potential penalties. Understanding the different categories of supplies and seeking professional advice can help you avoid these errors.

Failing to keep proper records is another frequent issue. To claim GST credits, you need valid tax invoices from your suppliers. If you lose or fail to obtain these invoices, you cannot claim the GST credits, increasing your tax bill. Using cloud-based accounting software can help you keep track of your invoices and records automatically.

Conclusion: Getting GST Registration Right from Day One

Understanding how GST works is essential for every Australian small business owner. The $75,000 threshold, the 21-day registration window, and the distinction between current and projected turnover are all critical concepts that can have significant financial consequences if misunderstood.

Whether you are just starting out or your business is growing rapidly, it is essential to monitor your turnover actively and seek professional advice when needed. A BAS agent or accountant can help you navigate the complexities of GST, ensure you register on time, and claim all eligible credits.

Unsure if you have crossed the threshold, or want help registering correctly? Our team at CA-Sir helps Australian small businesses and sole traders get GST registration right from day one. Contact us today to book a consultation and ensure your business is fully compliant with GST obligations.