What counts as GST turnover
Registering for GST isn't optional forever. Below the threshold it's a choice; at or above it, the Australian Taxation Office requires it. The number that decides it is your GST turnover, not your profit and not your take-home pay. GST turnover is your gross business income (the total value of what you sell), excluding GST itself, and excluding some categories the ATO treats differently, like input-taxed sales and certain other supplies. The ATO's own page on registering for GST sets out exactly what counts and what doesn't, and it's worth reading directly rather than relying on a summary, because the edge cases (private sales, GST-free supplies, disposals of business assets) can genuinely change the figure.
It's a rolling test, not an annual one
Here's the part that catches people out: it isn't a simple "check your income once a year" test. The ATO applies a rolling calculation, your turnover for the current month plus the previous eleven, and, separately, your projected turnover for the current month plus the next eleven. You're required to register the moment either of those crosses $75,000, whichever happens first. That means a business well under that figure on average can still tip over it because of a single unusually large job (a big fit-out, a large commercial contract, a run of work that lands in the same twelve-month window), even if a typical month wouldn't come close on its own. Projected turnover matters just as much as turnover already earned: if you can reasonably see a big contract coming that will push your rolling twelve months over the line, the obligation to register starts from when you could reasonably have expected that, not from the day the money actually lands.
Not-for-profits get a higher threshold
Not-for-profits get a different, higher threshold, recognising that many operate at a larger scale of turnover without the same profit motive as a standard business, $150,000, materially higher than the standard figure. It's worth checking on its own if that's your structure, because a not-for-profit reading the standard threshold would be registering, and potentially adding GST-handling overhead, well before it actually had to.
What actually changes once you register
What changes once you're registered is more than paperwork on a return. You start charging GST on your taxable sales, which for most trade and service businesses means adding it to every invoice from that point on. Quotes and pricing that didn't previously need to mention it now do. You take on a Business Activity Statement, usually lodged quarterly for a small business, reporting GST collected against GST credits claimed. And that second half is the part that gets underweighted in the "registration is a burden" framing: once registered, you can claim GST credits on your own business purchases (materials, fuel, tools, equipment), which for a business with real input costs can meaningfully offset the GST you're now charging. Our GST calculator covers the actual add/remove arithmetic once you're on either side of registration; this tool is about the threshold decision itself, not the ongoing calculation.
How GST turnover is actually worked out
GST turnover is not profit and it's not what lands in the bank after expenses. It's the gross value of what you sell, before costs come out, minus GST itself and a handful of specific exclusions. GST-free sales, basic food, most health services, exports, still count toward turnover even though no GST is charged on them. Input-taxed sales, residential rent and most financial supplies, generally don't count, and neither does the sale of a business asset you're disposing of rather than trading as part of normal turnover. Getting that distinction right matters more the closer a business sits to the threshold, because a large one-off asset sale sitting on the wrong side of the line can be the difference between having to register and not, even though it isn't really turnover in the ordinary sense of the word.
The 21-day countdown once you cross the line
Once your turnover crosses the threshold, whichever test trips it, current or projected, you have 21 days to register from the day you knew, or reasonably should have known, that you'd gone over. That start date isn't the day someone finally sits down and does the numbers. It's the day a reasonable business owner in your position would have seen it coming, which, for one very large job, might genuinely be the day the contract was signed rather than the day it was invoiced. Miss that window and the ATO can still require you to register, and can treat you as though you should have been registered from the date the obligation actually started, not from whenever registration is eventually sorted out.
Voluntary registration before you have to
Registering before you're required to is a genuine option, not just a formality for the record. It tends to make the most sense for a business buying a decent amount of equipment early on, tools, a vehicle, a fit-out, because voluntary registration lets you claim GST credits on those purchases from the date you register rather than from whenever you eventually cross the threshold anyway. It also tends to make more sense when most of your customers are other GST-registered businesses, since they can claim back whatever GST you charge them, so the extra 10% on your invoice doesn't actually cost them anything net. It makes less sense if your customers are mostly the public, price-sensitive, and comparing you against unregistered competitors, because for them the GST really is an extra 10%. There's no single right answer here. It depends on who buys from you and what you're spending establishing the business, which is exactly the kind of call worth running past an accountant before you commit to it.
What changes on day one of registration
Tax invoices carry their own formatting rules from day one, not just a GST line added to whatever you were already sending. Once a sale reaches $1,000 (GST inclusive), the invoice needs to show the buyer's identity or ABN as well as your own, on top of the usual requirements. Below that figure the buyer's details are optional. None of this is complicated once it's set up properly, but it's one more thing that has to be right on every invoice from the day registration starts, not something worked out retrospectively when the first BAS is due. See our tax invoice generator for the full requirements, and our markup and margin calculator for checking your margin still holds once GST is added to the price.
Registering late
If registration happens later than it should have, the ATO's position is that GST was still payable on the taxable sales made from the date you were required to register, whether or not you actually charged customers for it at the time. In practice that can mean finding the GST component out of money already spent, rather than money still sitting in the account waiting to be remitted, which is a considerably worse position than registering on time and pricing for it from the start. Interest and penalties can also apply on top of the GST itself. None of this is a reason to panic if you think you might be in this position. It's a reason to talk to your accountant promptly rather than wait for the ATO to raise it first, since catching it yourself is generally the better conversation. Businesses are required to keep most records, including the ones that would show exactly when a threshold was crossed, for 5 years, so the paperwork to work out the actual date is usually still there even if it wasn't front of mind at the time.
Where this fits with the rest of the admin
The practical risk isn't misunderstanding the rule. It's not noticing you've crossed the line until well after the fact. A good quarter, one large contract, a run of strong months, and turnover that used to sit comfortably below the threshold is suddenly over it, with the 21-day registration clock already running before anyone checked. The businesses that don't get caught out are the ones with a system that actually tracks turnover as it happens rather than reconstructing it from a shoebox of invoices in July. That's part of what we build under business systems, so a strong month shows up as a number you see coming, not a surprise your accountant delivers. It's also worth making sure your own pricing is clear about GST status before a customer ever calls. Check that against our pricing page and, if your site or Google listing quotes numbers, our web design work, so nobody's caught out by a mismatch between what was quoted and what was actually charged.