It's the week before BAS is due in a busy dental practice. Chair time is full. The practice manager has pulled a draft activity statement from the software. The GST figure looks "about right" compared with last quarter - and you're tempted to nod and move on between patients.
That nod is where a lot of small-business BAS mistakes start. Not because owners are careless, but because a staffed practice runs on appointments, not on tax labels. As at 16 September 2026, the Australian Taxation Office (ATO) publishes a short list of BAS and GST tips that map cleanly to the checks I want owners to make - or to ask their bookkeeper and registered tax agent - before anything is lodged.
This is a spotting guide. It is not a how-to lodge, claim, or revise your own BAS.
Why practice BAS goes sideways
Picture a legal or medical practice turning over about $2.4M with mixed taxable fees, some GST-free supplies, and a steady stream of supplier invoices for labs, software, and fit-out. Cash and accrual timing, invoice duplicates, and classification slips do more damage than exotic tax theory.
When owners tell me "the BAS felt fine until the ATO letter," the pattern is usually ordinary: the same purchase entered twice, a GST-free item coded as taxable (or the reverse), cents left in fields that expect whole dollars, or a cash-basis practice reporting invoices that haven't been paid yet.
The ATO's BAS and GST tips page (as at 16 September 2026) is blunt about several of those points. Whole dollars - leave the cents out, and don't round up. Enter each invoice once. Use the correct GST accounting method. If you're on a cash basis, timing follows cash. Only complete the fields that apply. Check formulas and classifications before you treat the draft as finished.
None of that requires you to become a BAS technician. It does require a short owner pass - or a clear ask of the person preparing the statement.
Six mistakes I see owners miss
1. Cents and rounding habits. Software sometimes shows dollars and cents. ATO BAS tips (as at 16 September 2026) say to report in whole dollars, leave cents out, and not round up. If your draft still shows cents in every field, that's a question for whoever prepared it - not a DIY rewrite mid-appointment.
2. The same invoice twice. Double counting purchases is a classic. A lab invoice imported from the bank feed and keyed from a PDF looks like diligence. On the BAS it can look like a claim that doesn't match reality. ATO material on fixing BAS mistakes (as at 16 September 2026) lists double counting purchases among the examples that sit in the error-and-adjustment world.
3. GST-free vs taxable mixed up. Practices often have a blend: taxable clinical or professional fees beside GST-free or special categories depending on the supply. Misclassification either way can overstate or understate GST. The ATO cites GST-free versus taxable misclassification as a common mistake pattern (as at 16 September 2026). Spotting it means asking which supplies were reviewed - not guessing codes yourself under time pressure.
4. Wrong GST method or cash timing. If the practice accounts for GST on a cash basis, unpaid invoices generally shouldn't drive the same GST rhythm as a cash receipt. ATO tips (as at 16 September 2026) flag using the correct GST method and, for cash accounting, getting the timing right. If your debtors book is large and the GST on sales still looks like a full accrual month, ask why.
5. Fields completed "because the software asked." Only complete the labels that apply to your reporting. Extra noise in unused fields is a common software-driven slip. ATO tips (as at 16 September 2026) say to complete only the fields that apply.
6. Formulas and classifications nobody re-checked. Spreadsheet bridges, rule-based bank feeds, and recycled coding from last year all drift. ATO tips (as at 16 September 2026) include checking formulas and classifications. In a practice with three people touching bills, drift is normal - silent review is the risk.
Error vs adjustment - what to ask, not what to DIY
As at 16 September 2026, the ATO distinguishes fixing BAS mistakes from making adjustments, and notes that some issues may be corrected on a later BAS while others may need a revision - conditions apply. Examples on that page include clerical errors, GST-free versus taxable misclassification, and double counting purchases.
Separately, ATO guidance on types of GST errors (as at 16 September 2026) talks in terms of credit errors and debit errors. That distinction matters for how a correction is handled under the rules - and it is exactly the sort of detail your tax agent should apply to your facts. I am not going to prescribe which correction path belongs on your next statement. The owner job is to notice the mismatch and ask the right person which path applies.
If something looks off after lodgement, the useful questions are calm ones: Was this an error or an adjustment under ATO language? Does it need a revision, or can it wait for the next BAS under the conditions the ATO describes? Who owns the paper trail?
Owner checklist before anyone lodges
Use this as a conversation list with your bookkeeper or tax agent - not as a self-lodgement script:
- Are all BAS figures in whole dollars, with cents left out and no rounding up (ATO tip, as at 16 September 2026)?
- Has each supplier invoice and sales invoice been entered once - no bank-feed plus PDF doubles?
- Which supplies were checked as taxable vs GST-free (or other categories), and who signed off?
- Are we on the GST method we think we are - and if cash basis, does timing follow cash?
- Are only the fields that apply completed?
- Have formulas, feed rules, and classifications been spot-checked for this period?
- If something looks wrong from a prior period, have we asked whether it's an error or an adjustment, and what the ATO options are - without guessing the path?
You're aiming for one clear outcome: I understand what this draft is saying, and I've asked about the bits that look soft.
What to do next
If BAS week in your practice still feels like a trust exercise with a PDF, that's useful information. Most owners I work with don't need more portals. They need a short, repeatable check before lodgement - and a tax agent or bookkeeper who can explain error versus adjustment language when something slips.
If you want help tightening that owner-side review for your practice, get in touch with me at Full Cycle Accounting and we can walk through what your draft is actually showing.