Businesses registered for GST generally report their GST obligations through a BAS or, where eligible, an annual GST return. A BAS can include GST, PAYG withholding and other tax obligations that apply to the business. Getting your BAS preparation right matters, because errors can lead to corrections, interest or penalties depending on the circumstances. This guide walks through the due dates, the most common mistakes, and a simple checklist you can use before every BAS lodgement.
Quick Answer
- Standard quarterly BAS due dates are 28 October, 28 February, 28 April and 28 July. Monthly BAS is generally due on the 21st of the following month. A registered agent can sometimes get you extra time.
- Lodging late can cost you a Failure to Lodge penalty. From 1 July 2026, this can add up to $1,820 for a small business.
- The eight mistakes below cause most of the BAS problems we see. Each one has a simple fix.
- Even a quiet quarter with no sales still needs a nil BAS lodged on time.
WHY IT MATTERS
Why BAS lodgement mistakes cost more than the fine on the letter
A late or wrong BAS lodgement is rarely a small problem. The Failure to Lodge penalty grows every 28 days. Interest keeps adding up on any tax you still owe. Repeated late lodgement can also increase the risk of ATO compliance action.
| Business size | Penalty units per 28 days | Most blocks charged | Biggest fine (from 1 Jul 2026) |
|---|---|---|---|
| Small entity — an entity not classified as medium or large | 1 unit | 5 | $1,820 |
| Medium entity — generally assessable income/current GST turnover over $1m and under $20m, or a medium PAYG withholder | 2 units | 5 | $3,640 |
| Large entity — generally assessable income/current GST turnover of $20m or more, or a large PAYG withholder | 5 units | 5 | $9,100 |
CALENDAR
BAS due dates for 2026–27
Some businesses lodge every quarter. Others lodge every month. You may be eligible for additional time when lodging through a registered tax or BAS agent. Mixing these dates up is mistake number one below, so keep this table handy.
| Quarter | Period covered | Standard due date | Online self-lodge concession |
|---|---|---|---|
| Q1 | 1 Jul – 30 Sep 2026 | 28 Oct 2026 | 11 Nov 2026 |
| Q2 | 1 Oct – 31 Dec 2026 | 28 Feb 2027 (falls on a Sunday — effective due date is Monday, 1 March 2027) | No extra time — already extended |
| Q3 | 1 Jan – 31 Mar 2027 | 28 Apr 2027 | 12 May 2027 |
| Q4 | 1 Apr – 30 Jun 2027 | 28 Jul 2027 | 11 Aug 2027 |
| Monthly lodgers | Each calendar month | 21st of the next month | — |
| Annual GST return | Full financial year | 31 Oct 2026 (28 Feb 2027 if you don't lodge a tax return) | — |
THE COMMON SLIP-UPS
8 BAS lodgement mistakes that keep showing up
None of these mean a business is doing anything wrong on purpose. Most happen because the business grew faster than its bookkeeping habits. Here is each mistake in plain English, with the fix we would actually use.
Missed Deadline
Mixing up the different due dates
Three due dates can apply to the same quarter: the standard date, the online concession date, and an agent’s later date. Some businesses use the wrong one and lodge weeks late without realising it.
Fix
Write down only the due date that matches how you actually lodge, not the latest one you have seen mentioned somewhere.
Wrong Method
Reporting GST the wrong way – cash vs accruals
Your BAS must use the method you are registered for with the ATO: cash or accruals. Businesses with a turnover under $10 million can usually choose cash, which counts GST when money actually changes hands. Switching by accident, or letting software default to the wrong setting, makes your numbers not match your tax return.
Fix
Check your registered method before your first BAS each year, and lock it into your accounting software’s settings.
Over-Claimed
Claiming GST credits you should not
Some entertainment expenses, fines and penalties, and purchases without the required supporting documentation are common problem areas. The ATO matches your BAS against your tax return and other reports, so these claims are easy to spot.
Fix
Check your expense list every quarter before you lodge, and keep appropriate records for GST credits you claim. For purchases over $82.50 including GST, you generally need to hold a valid tax invoice.
Unreconciled
Lodging before your bank accounts are checked
Duplicate transactions and wrongly coded bank transfers can push your GST figures up or down. This is the single biggest reason we see BAS statements that need fixing later.
Fix
Match every bank account down to zero difference before you touch the lodgement screen, not after.
PAYG Slip-Up
Getting staff tax amounts wrong
Old tax tables, unfinished payroll data, or mixing up a contractor with an employee can all throw off the wage-tax figures on your BAS.
Fix
Update your payroll software before each BAS, and check the numbers against your payroll reports.
Asset GST Gap
Forgetting GST when you sell an asset
Selling a work vehicle or old equipment often triggers GST that is easy to miss if nobody tells the person preparing your BAS.
Fix
Tell your bookkeeper about any asset sale the moment it is agreed, not at the end of the quarter.
Nil, Not Skipped
Thinking a quiet quarter means no lodgement
If you are registered for GST, you must still lodge a nil BAS when you have had no sales or purchases. The ATO usually won’t fine a small business for a late nil result on its own. But it’s still a legal obligation, and repeated late lodgement can bring closer scrutiny.
Fix
Lodge the nil BAS through the same method and on the same date as any other quarter, so it never becomes a pattern the ATO notices.
DIY Drift
Doing it yourself once your business has outgrown that
Once you have staff in several states, multiple income types, or extra taxes like fuel tax credits, a simple spreadsheet is often not enough anymore.
Fix
Consider using a registered BAS or tax agent once your reporting becomes more complex, particularly where multiple tax obligations apply.
Don't let a missed BAS deadline cost you up to $1,820.
BEFORE YOU LODGE
A simple 7-point BAS checklist
Run through this BAS preparation checklist before every lodgement, no matter who is actually pressing submit.
- Bank and credit card accounts matched to the cent
- GST codes checked on every transaction, not just the big ones
- Private and entertainment costs left out of GST credits
- Payroll finished and matched to your wage-tax figures
- Any asset sales checked for the right GST treatment
- Reporting method (cash or non-cash/accrual) matches the method currently used for your GST reporting
- Due date confirmed for how you actually lodge, yourself or through an agent
CONCLUSION
Almost every mistake in this BAS lodgement guide comes back to the same root cause. BAS preparation starts on the day it’s due, instead of the week before. Businesses that stay free of penalties are not always the ones with the simplest books. They are the ones with a steady reconciliation habit and a clear answer to “which due date applies to us?”.
If your BAS process still runs on memory and a spreadsheet, 2026’s higher penalty amounts make this the year to tighten it up. You can use the checklist above yourself, or let a registered agent handle the calendar for you. AAD Taxation is a Tax Practitioners Board–registered firm with offices across Victoria, New South Wales, Queensland and Tasmania, helping local businesses lodge their BAS correctly and on time, every quarter.
Frequently asked questions
How often do I need to lodge a BAS?
Most businesses lodge every quarter. If your GST turnover is $20 million or more, you must lodge every month instead. Businesses that are voluntarily registered for GST and meet the ATO’s turnover requirements may be eligible to report GST annually.
What happens if I lodge my BAS late?
The ATO can charge a Failure to Lodge penalty. It is one penalty unit for every 28 days you are late, up to five units. From 1 July 2026, one penalty unit is $364, so a small business can be fined up to $1,820 for one late BAS.
Can I fix a mistake after I have already lodged my BAS?
Yes. Some GST errors can be corrected on a later BAS if the ATO’s eligibility conditions, time limits and applicable value limits are met. Otherwise, you may need to revise the earlier BAS. Fix mistakes as soon as you find them, because they can carry over into later statements.
Should I lodge my own BAS or use a registered agent?
If your business is simple, you may be able to lodge it yourself. Once you have staff, sell assets, or run more than one type of income, a registered BAS or tax agent usually saves you more than they cost, mainly by helping you avoid mistakes.
Do I still need to lodge a BAS if I had no sales?
Yes. If you are registered for GST, you must still lodge a nil BAS, even in a quiet quarter. The ATO generally does not fine a small business just for a late nil result. But it’s still a legal requirement, and a pattern of late lodgement can bring closer attention from the ATO.