How to Lodge Your BAS as an Australian Sole Trader: GST, PAYG Instalments and the Mistakes That Cost Thousands (2026)

If you're an Australian sole trader registered for GST — or approaching the $75,000 turnover threshold — you need to lodge a Business Activity Statement (BAS) with the ATO every quarter. Get it right and it's a straightforward 20-minute task. Get it wrong and you're looking at penalties, interest charges, and ATO scrutiny that can snowball fast. This guide explains exactly what goes on your BAS, how to prepare it, the deadlines that matter, and the mistakes that cost sole traders thousands every year.

What Is a BAS and Who Needs to Lodge One?

A Business Activity Statement is the form you use to report and pay several tax obligations to the ATO, including GST collected and paid, PAYG instalments on your business income, and PAYG withholding if you have employees. For most sole traders, the BAS is primarily about GST.

You must register for GST — and therefore lodge a BAS — if your business has a GST turnover of $75,000 or more per year (or $150,000 for non-profit organisations). You can also register voluntarily below that threshold, which lets you claim GST credits on business purchases.

Even if you're not GST registered, you may still need to lodge a BAS if the ATO has placed you on PAYG instalments for your income tax. This typically happens after your first tax return shows business income — the ATO will send you a letter with your instalment rate or amount.

What Goes on Your BAS: The Four Key Labels

The BAS form can look intimidating with dozens of labels, but most sole traders only need to worry about four:

Your GST payable for the quarter is Label 1A minus Label 1B. In the example above: $5,000 − $1,800 = $3,200 owed to the ATO.

BAS Lodgement Deadlines (Quarterly)

Most sole traders lodge quarterly. Here are the deadlines:

If the due date falls on a weekend or public holiday, you have until the next business day. Using a registered tax agent gives you an automatic extension on most quarters — but Q2 (the December quarter) has the same deadline regardless.

A Worked BAS Example: Freelance Web Developer

Let's walk through a real quarterly BAS for a sole trader web developer earning $120,000 per year (GST-inclusive).

Income for the quarter

• Invoice 1: Website build — $8,800 (including $800 GST)
• Invoice 2: Monthly retainer × 3 — $6,600 (including $600 GST)
• Invoice 3: Consulting — $4,400 (including $400 GST)
• Invoice 4: E-commerce project — $13,200 (including $1,200 GST)

Total sales (G1): $33,000
Total GST collected (1A): $3,000

Business expenses for the quarter

• Web hosting & domains — $330 (including $30 GST)
• Software subscriptions (Figma, GitHub, etc.) — $550 (including $50 GST)
• New monitor — $1,100 (including $100 GST)
• Home office internet (business portion 60%) — $264 (including $24 GST)
• Accounting software — $66 (including $6 GST)
• Professional development course — $440 (including $40 GST)

Total purchases (G11): $2,750
Total GST on purchases (1B): $250

The BAS calculation

That $6,350 is due by the quarterly deadline. If you've been setting aside 30–35% of each payment into a separate tax account (as recommended in our cash flow management guide), this shouldn't be a surprise. If you haven't, it's a painful quarter.

PAYG Instalments: The Part That Catches People Off Guard

Many sole traders think the BAS is only about GST. It's not. After your first tax return shows business income, the ATO will typically put you on PAYG (Pay As You Go) instalments — quarterly prepayments towards your expected income tax bill.

There are two methods:

• Instalment rate method (T7/T8): The ATO gives you a percentage rate (say, 12%). You multiply it by your quarterly business income to calculate your instalment. This adjusts automatically if your income goes up or down.
• Instalment amount method (T9): The ATO tells you a fixed dollar amount to pay each quarter, based on your previous tax return. This stays the same regardless of income fluctuations.

Most sole traders are better off using the instalment rate method — it means you pay less in quiet quarters and more in busy ones, which is easier on cash flow. You can vary your instalment rate or amount if your income changes significantly, but be careful: if you under-estimate and underpay by more than a certain threshold, the ATO charges interest.

How to Prepare Your BAS: Step by Step

Step 1: Reconcile your bank accounts

Before touching the BAS, make sure every transaction in your business bank account is categorised and reconciled. In accounting software like Xero or MYOB, this means matching every bank feed item to an invoice, expense, or transfer. In a spreadsheet, it means ensuring every row in your income and expense tracker matches a bank transaction.

Step 2: Check your GST codes

Every transaction needs the correct GST treatment:

• GST (10%): Most sales and business purchases
• GST-free: Basic food, medical services, exports, some education
• Input taxed: Financial supplies, residential rent (no GST credits)
• No ABN withholding: If a supplier hasn't quoted an ABN, you may need to withhold 47% from the payment

The most common error is coding personal expenses as business expenses with GST credits. The ATO data-matches your claims against industry benchmarks — if your GST credits look high relative to your income, expect a review.

Step 3: Review your sales and income

Make sure all invoices issued during the quarter are recorded. If you report on a cash basis (most sole traders do), only include income you actually received during the quarter — not invoices that are still unpaid. If you report on an accruals basis, include all invoices issued regardless of payment.

Step 4: Generate the BAS report

If you use accounting software, it generates the BAS figures automatically. Run the BAS report, review the numbers, and compare them to the previous quarter for any unusual movements. If the GST collected is dramatically different from previous quarters, investigate before lodging.

Step 5: Lodge and pay

Lodge online through the ATO Business Portal, myGov (linked to your ABN), or through your accounting software (Xero, MYOB, and QuickBooks can lodge directly). Pay via BPAY, direct debit, or credit/debit card (card payments attract a surcharge).

Cash vs Accruals Reporting: Which Should You Use?

You need to choose how you report GST on your BAS:

If your aggregated turnover is under $10 million, you can choose either method. Cash basis is almost always better for sole traders because you don't have to pay GST to the ATO on invoices your clients haven't paid yet. Given that late payment is the norm in many industries, this can make a significant difference to your cash flow.

Monthly vs Quarterly vs Annual Reporting

Most sole traders lodge quarterly, but you have options:

• Quarterly: The default for most businesses. Lodge and pay four times a year.
• Monthly: Optional if you want more frequent GST refunds (useful if you regularly claim more GST credits than you collect — e.g., export businesses). Must lodge by the 21st of the following month.
• Annual: Only available if your GST turnover is under $75,000 and you voluntarily registered. You still need to pay quarterly PAYG instalments if applicable.

What Happens If You Lodge Late or Don't Pay

The ATO takes BAS obligations seriously. Here's what you're facing:

The GIC is the one that really hurts. At ~11% per annum accruing daily, a $10,000 unpaid BAS debt grows by about $3.11 per day. Over six months, that's an extra $565 in interest on top of the original amount.

The 7 Most Common BAS Mistakes (and How to Avoid Them)

1. Claiming GST credits on expenses that don't include GST

Not every business expense includes GST. Common GST-free items that sole traders incorrectly claim credits on:

• Bank fees and interest charges
• Insurance premiums (many are input-taxed)
• Government fees (ASIC fees, some council charges)
• Wages and superannuation payments
• Purchases from suppliers not registered for GST

Always check the tax invoice. If there's no GST component listed, you can't claim a GST credit.

2. Including personal expenses in business GST claims

As a sole trader, there's no legal separation between you and your business. This makes it easy to blur the line between personal and business expenses. If an expense is partly personal — like a phone or car — you can only claim the business-use percentage.

3. Not keeping valid tax invoices

To claim a GST credit, you must hold a valid tax invoice from the supplier. For purchases over $82.50 (including GST), the invoice must show the supplier's ABN, the GST amount, and the date. A bank statement alone is not a tax invoice. The ATO can disallow GST credits if you can't produce the invoice during an audit.

4. Reporting income on the wrong basis

If you're on cash basis, you report income when you receive payment — not when you issue the invoice. Reporting the wrong way means you could be paying GST to the ATO before your client has even paid you, creating unnecessary cash flow pressure.

5. Forgetting to include all income sources

If you receive income from multiple clients, platforms, or sources (e.g., Upwork payments, direct invoices, and ad revenue), all GST-applicable income must be reported on your BAS. The ATO cross-references your BAS figures with your annual tax return and third-party data — discrepancies trigger automated review.

6. Not adjusting PAYG instalments when income changes

If your business income drops significantly but you keep paying the same PAYG instalment, you're lending the ATO money interest-free for months. Conversely, if your income jumps and you don't increase your instalment, you'll face a large tax bill at the end of the year plus potential interest. You can vary your instalment rate at any time through the ATO portal.

7. Leaving the BAS until the last minute

Scrambling to reconcile three months of transactions the night before the deadline is how errors happen. If you reconcile weekly or fortnightly, the quarterly BAS becomes a 20-minute review rather than a stressful catch-up. Set a recurring calendar reminder for the first of each month to reconcile the previous month.

Should You Use Accounting Software or Do It Manually?

If you're GST registered, accounting software is almost always worth it. The $300–$720 annual cost is fully tax deductible and will save you hours of manual calculation while dramatically reducing errors. If your quarterly BAS is consistently straightforward (few transactions, simple income), a BAS agent review once a year plus self-lodgement can work.

The GST Registration Decision: Should You Register Voluntarily?

If your turnover is under $75,000, you're not required to register for GST. But should you?

Arguments for voluntary registration

• You can claim GST credits on business purchases — recovering 10% of the GST component
• Your business appears more established and professional to clients
• If you're approaching the $75,000 threshold, registering early avoids the scramble of back-registering
• You're forced to keep better records from the start

Arguments against voluntary registration

• You must charge GST on your services — if your clients are consumers (not businesses), they can't claim the GST back, making you 10% more expensive
• Quarterly BAS lodgement is an administrative burden
• If you mostly sell to the public (B2C), GST registration effectively reduces your revenue by 1/11th unless you increase prices

BAS Amendments: Fixing Mistakes After Lodging

Made an error on a BAS you've already lodged? You have two options:

• Revise the original BAS: You can revise a previously lodged BAS through the ATO Business Portal or your accounting software. This is appropriate for any error you discover.
• Correct on the next BAS: For GST errors under $7,500, the ATO allows you to correct the mistake on your next BAS rather than amending the original. This is simpler but only works for smaller adjustments.

Self-correcting errors shows the ATO you're acting in good faith. It's far better to fix a mistake yourself than to have the ATO find it during a review.

A Quarterly BAS Checklist for Sole Traders

Use this checklist every quarter to make sure nothing is missed:

1. Reconcile all bank transactions for the quarter in your accounting software or spreadsheet
2. Check GST coding on every transaction — correct any that are wrong
3. Verify all invoices issued during the quarter are recorded
4. Confirm cash vs accruals basis — are you reporting income/expenses at the right time?
5. Review GST credits — do you have a valid tax invoice for every claim?
6. Check PAYG instalment amount or rate — does it still reflect your current income?
7. Compare figures to the previous quarter — any unusual movements to investigate?
8. Generate and review the BAS report from your accounting software
9. Lodge the BAS through the ATO portal, myGov, or accounting software
10. Pay the amount owing via BPAY or direct debit before the deadline

The Bottom Line

The BAS is not complicated — it just punishes disorganisation. If you reconcile your accounts regularly, keep valid tax invoices, and set aside money for GST and PAYG instalments as you earn it, each quarterly BAS becomes a routine 20-minute job rather than a stressful, error-prone scramble.

The most important things to remember:

• Set aside 30–35% of every payment into a separate tax account — this covers both GST and income tax instalments
• Reconcile your accounts weekly or fortnightly, not the night before the BAS is due
• Use cash basis reporting so you only pay GST after you've been paid
• Keep every tax invoice — the ATO can disallow GST credits without them
• Review your PAYG instalment rate each quarter and vary it if your income has changed significantly
• Contact the ATO before the deadline if you can't pay — a payment plan beats the GIC every time
• Invest in accounting software if you're GST registered — the cost is tax deductible and the error reduction alone pays for it