How to Do Your Own BAS: Step-by-Step Guide

Published April 2026 • 9 min read

The Business Activity Statement strikes fear into many Australian small business owners. It arrives every quarter (or month), demands numbers you may not have ready, and carries penalties if you get it wrong or lodge it late. But here is the truth: BAS is not that complicated once you understand what it is asking for.

This guide breaks down the entire BAS lodgement process into clear, manageable steps. Whether you are a sole trader filing your first BAS or a small business owner who has been guessing for too long, this article will give you the confidence to lodge accurately and on time -- without paying an accountant hundreds of dollars each quarter.

What Is a BAS?

A Business Activity Statement is a form you submit to the Australian Taxation Office to report and pay several tax obligations, including Goods and Services Tax (GST), Pay As You Go (PAYG) income tax instalments, PAYG withholding (tax withheld from employee wages), and Fringe Benefits Tax (FBT) instalments.

Think of the BAS as a regular check-in with the ATO. You are telling them how much GST you collected, how much you paid, how much tax you withheld from employees, and settling the balance. For most small businesses, GST is the primary component, and that is where we will focus most of our attention.

Who Needs to Lodge a BAS?

You must lodge a BAS if your business is registered for GST. GST registration is mandatory once your annual turnover reaches $75,000 ($150,000 for non-profit organisations). If you are voluntarily registered for GST below these thresholds, you also need to lodge.

Even if you had no business activity during a period, you must still lodge a "nil" BAS. Failing to lodge -- even a nil return -- can result in penalties and interest charges. The ATO does not assume silence means zero activity.

Quarterly vs Monthly Lodgement

Most small businesses lodge quarterly. The ATO financial quarters are:

• Q1: July -- September (due 28 October)
• Q2: October -- December (due 28 February)
• Q3: January -- March (due 28 April)
• Q4: April -- June (due 28 August)

Monthly lodgement is required if your annual turnover exceeds $20 million, or you can opt in voluntarily. Monthly BAS is due on the 21st of the following month. For most small businesses, quarterly lodgement is simpler and perfectly adequate.

If you use a registered tax agent to lodge, you may be eligible for extended deadlines. Check with your agent or the ATO for the current schedule.

Step-by-Step: Lodging Your BAS

Here is the process from start to finish. Set aside about an hour for your first BAS -- it gets much faster once you have done it a few times.

Step 1: Gather Your Records

Before you start, collect all financial records for the BAS period:

• Sales invoices you issued (with GST amounts)
• Expense receipts and supplier invoices (with GST amounts)
• Bank statements for the period
• Payroll records if you have employees
• Any adjustments from previous BAS periods

Step 2: Reconcile Your Accounts

Match your bank statements against your invoices and receipts. Every transaction should be accounted for. This is the step most people skip, and it is the primary cause of BAS errors. If you track expenses in a tool like BizziKit Financial Accounting Tools, this reconciliation is largely done for you.

Step 3: Calculate GST Collected (Label 1A)

Add up the GST component of every taxable sale you made during the period. If you sold $11,000 worth of goods (GST-inclusive), the GST collected is $1,000. This is the amount you report at Label 1A.

Step 4: Calculate GST Paid (Label 1B)

Add up the GST you paid on business purchases during the period. Only include purchases where you have a valid tax invoice. This is the amount you report at Label 1B.

Step 5: Work Out Your GST Payable or Refund

Subtract 1B from 1A. If the result is positive, you owe the ATO. If negative, the ATO owes you a refund. For example, if you collected $5,000 GST on sales and paid $3,200 GST on purchases, you owe $1,800.

Step 6: Report PAYG Withholding (if applicable)

If you have employees, enter the total tax you withheld from their wages during the period. This figure should match your payroll records and the amounts reported through Single Touch Payroll.

Step 7: Lodge via myGov or Your Tax Agent Portal

Log into your myGov account linked to the ATO, navigate to the BAS section, enter your figures, review for accuracy, and submit. You will receive a confirmation and a payment reference number if you owe money.

How to Calculate GST for Your BAS

There are two methods the ATO allows for calculating GST, and you must choose one when you register:

Cash Basis (Simpler)

You only report GST when money actually changes hands. If you invoiced a customer in March but they paid in April, the GST goes on the April quarter BAS. This method is simpler for cash flow management and is available to businesses with turnover under $10 million.

Accruals Basis

You report GST when you issue or receive an invoice, regardless of when payment occurs. The March invoice goes on the March quarter BAS even if payment arrives in April. This method is required for businesses with turnover over $10 million and is often used by larger small businesses for more accurate financial reporting.

For most sole traders and micro-businesses, the cash basis is simpler and better for cash flow management. You do not pay GST to the ATO until your customer has actually paid you.

Understanding PAYG Withholding

If you employ staff, PAYG withholding is a mandatory part of your BAS. This is the tax you deduct from employee wages before paying them. The amount to withhold is determined by ATO tax tables based on each employee's earnings and tax file number declaration.

With Single Touch Payroll (STP) now mandatory for all employers, your payroll software reports withholding amounts to the ATO each pay cycle. The BAS total should match the sum of your STP reports for the period. If there is a discrepancy, investigate before lodging.

Common BAS Mistakes to Avoid

1. Claiming GST on GST-Free Items

Not everything you buy has GST. Basic food items, some medical expenses, and certain financial services are GST-free. Do not claim input tax credits on purchases that did not include GST -- check your supplier invoices carefully.

2. Forgetting to Include All Sales

Every taxable sale must be reported, including cash sales, online sales, and sales through third-party platforms. The ATO cross-references data from multiple sources, so underreporting is likely to be detected.

3. Mixing Personal and Business Expenses

Only claim GST credits on legitimate business purchases. If an expense is partly personal (like a phone used 50% for business), only claim the business portion. Overclaiming is a common audit trigger.

4. Claiming Without Valid Tax Invoices

You need a valid tax invoice to claim input tax credits on purchases over $82.50. A plain receipt is not sufficient -- it must include the supplier's ABN, the GST amount, and other required fields. No tax invoice means no GST credit.

5. Lodging Late or Not at All

Late lodgement penalties start at $313 per 28-day period and can compound quickly. Even if you cannot pay the amount owed, lodge on time and then arrange a payment plan with the ATO. The penalty for non-lodgement is separate from and additional to any penalty for late payment.

ATO Deadlines and Penalties

The ATO takes BAS deadlines seriously. Here is what you need to know:

• Quarterly BAS: Due on the 28th of the month following the end of the quarter (except Q2, which is due 28 February)
• Monthly BAS: Due on the 21st of the following month
• Late lodgement penalty: Starts at one penalty unit ($313 in 2026) for each 28-day period the BAS is overdue, up to a maximum of five penalty units
• Late payment interest: The General Interest Charge (GIC) applies to unpaid amounts from the due date. The rate is updated quarterly by the ATO

If you are struggling to pay, contact the ATO before the due date to arrange a payment plan. They are generally willing to work with businesses that communicate proactively, but far less lenient with those who simply ignore their obligations.

Doing your own BAS is entirely achievable for most small businesses. The key is staying organised throughout the quarter rather than scrambling at deadline time. Track your income and expenses as they happen, keep your tax invoices filed, and reconcile regularly. When BAS time comes, you will have everything ready and the process will take minutes instead of hours.