How to Do a BAS as a Sole Trader: Step by Step (G1, 1A, 1B)

Short answer: To do a BAS as a GST-registered sole trader, enter every sale you made in the period, including GST, at G1, the GST in your taxable sales at 1A, and the GST in your business purchases at 1B. If the ATO has put you on PAYG instalments, the instalment goes at 5A. What you owe minus what you are owed is the amount to pay or the refund. Lodge online, through software or a registered agent, and pay by the due date quoting your payment reference number.

A business activity statement is how a GST-registered business reports and pays GST, and for many sole traders it also carries PAYG income tax instalments. Most sole traders are on Simpler BAS, the ATO’s default for businesses with GST turnover under $10 million, which reports just three GST labels. This guide walks through one quarter of the 2026–27 income year. New to the idea? Read what is a BAS first; for when each one is due, see BAS due dates.

What do you need before you start your BAS?

Your sales and purchase records for the period, your GST accounting basis, and any PAYG instalment amount or rate the ATO has given you.

• Sales: every invoice you issued, or payment you received, in the period, checked against your bank statements.
• Purchases: a tax invoice for each business purchase you will claim GST on, with the private share taken out. See expenses and receipts.
• Accounting basis: cash or accruals, which decides the period each item falls in.
• PAYG instalments: the amount or rate shown on your activity statement.

Under Simpler BAS there is no GST calculation worksheet, and on a paper statement the sections you do not need can be left blank.

Cash or accruals: which period does each sale belong in?

On a cash basis a sale is reported when you are paid; on an accruals basis, when you issue the invoice or are paid, whichever comes first.

Situation	Cash basis	Accruals basis
You invoice a job on 26 September; the client pays on 9 October	Oct–Dec quarter	Jul–Sep quarter
A supplier invoices you on 28 June; you pay on 5 July	Jul–Sep quarter	Apr–Jun quarter
Cash sale on the day	Same quarter either way	Same quarter either way

Purchases follow the same rule. Cash accounting suits a sole trader who waits weeks to be paid, because you never pay GST on money you have not yet received. Eligibility depends on your turnover; the ATO sets out the test on its choosing an accounting method page.

How do you fill in G1 total sales?

G1 is the total of all the sales you made in the period, with GST included.

That means taxable sales at their GST-inclusive price, plus any GST-free sales, which carry no GST but still count. It does not include money that is not a sale: cash you put into the business, loans, or wages from a job. The paper form asks whether G1 includes GST; answer yes. As a check, G1 should match your sales invoices for the period on accruals, or what customers paid you on cash.

How do you work out 1A and 1B?

1A is the GST in your taxable sales; 1B is the GST in your business purchases that you are entitled to claim.

GST is 10% of the price before GST, so the GST inside a GST-inclusive amount is one-eleventh of it. 1A is one-eleventh of your taxable sales. If everything you sell is taxable, that is one-eleventh of G1; if some sales are GST-free, take them out first, because they carry no GST. How to calculate GST has more worked examples.

1B is one-eleventh of the GST-inclusive cost of your business purchases that include GST. Leave out:

• the private share of anything used partly for private purposes
• purchases with no GST in them, such as bank fees, wages, super, or anything from a supplier not registered for GST
• anything you cannot back up with a tax invoice or other record the ATO accepts

Your sales invoices drive 1A, so they have to be right; how to invoice as a sole trader covers what a tax invoice must show.

What goes at the PAYG instalment labels?

If you are on PAYG instalments, you either pay the amount the ATO has worked out or multiply your instalment income by the ATO’s rate, and the result goes to 5A.

Option	Label	What goes there
1 (amount)	T7	The instalment amount the ATO has set for the period
1, varied	T8, T9, T4	Your estimated tax for the year, the varied amount for the period, and a reason code
2 (rate)	T1	Your instalment income for the period: gross business and investment income, excluding GST
2 (rate)	T2	The instalment rate the ATO gave you
2, varied	T3, T4	Your new rate, and a reason code
2 (rate)	T11	T1 × T2 (or × T3 if varied)
Both	5A	The PAYG instalment you are paying for the period

Option 1 is less work; option 2 tracks your income, so a quiet quarter costs less. You can vary an instalment if this year will differ sharply from last, but a variation that proves too low can attract interest. In your first year there is usually no instalment, because the ATO has no return to base it on; sole trader taxes explains how instalments are credited at tax time. If you employ staff, the withholding labels apply too: W1 for wages paid and W2 for tax withheld.

Worked example: a quarterly BAS for a sole trader

Sam is a sole trader electrician on Simpler BAS, cash basis, option 1 instalments. For July–September 2026 he received $33,000 from customers, all for taxable work, and paid $9,900 including GST on business purchases: $8,250 of materials, $990 of fuel, $440 for a tool and $220 for the business share of his phone and internet. A $3,800 job he invoiced on 26 September is paid in October, so it waits for the next BAS.

Label	What it is	Amount
G1	Total sales, including GST	$33,000
1A	GST on sales ($33,000 ÷ 11)	$3,000
1B	GST on purchases ($9,900 ÷ 11)	$900
T7 → 5A	PAYG instalment set by the ATO	$1,450
8A	Total you owe the ATO (1A + 5A)	$4,450
8B	Total the ATO owes you (1B)	$900
9	Amount to pay (8A − 8B)	$3,550

When 8B is larger than 8A, you get a refund instead.

How do you lodge your BAS online?

Most sole traders lodge through ATO online services linked to myGov, through accounting software that lodges directly, or through a registered BAS or tax agent.

1. Open the activity statement for the period in ATO online services, or in your software.
2. Enter G1, 1A and 1B, then the instalment section if it applies.
3. Check the summary: what you owe, what you are owed, and the net amount.
4. Lodge it, and keep the confirmation with your records for the period.

With no sales and no purchases, you still lodge, with nil amounts. Clients of a registered agent may have different dates; BAS due dates sets those out, and the Australian guides collect the rest.

How do you pay your BAS?

Pay the net amount by the due date, quoting the payment reference number (PRN) shown on your statement.

The PRN links your payment to your account; without it, a payment can sit unallocated. The ATO accepts several methods, including BPAY and card. If you cannot pay in full, lodge on time anyway and ask the ATO about a payment plan, though interest builds on anything unpaid. The habit that makes BAS painless is moving one-eleventh of every taxable payment you receive into a separate account the day it arrives.

What if you made a mistake on your BAS?

Many GST errors can be fixed in a later BAS; others need the original statement revised.

The ATO lets you correct some GST errors on a later activity statement, within time and value limits that depend on your turnover, as explained on its BAS pages. Outside those limits, revise the BAS for the original period through online services or your agent. Keep a note of what changed and why with the corrected invoice or receipt, and tell the ATO before it asks: penalties are generally lower when you come forward first.

…and where does Keel fit?

Keel does not lodge your BAS, connect to the ATO or work out what you owe. It keeps the records a BAS is built from.

Keel: Invoice Maker & Receipts, by Ilura Technology OÜ, is an iPhone app for people who work for themselves. Your invoices (G1 and 1A) and your receipts and expenses (1B) sit under each job, and “who owes you” shows which invoices are still unpaid, which matters on a cash basis. Records stay on the iPhone: no account, no bank connection, no cloud sync, and the App Store privacy label reads “Data Not Collected”. Keel is free with no invoice limit; Keel Lifetime, a one-time purchase ($249.99 USD, shown in local currency on the App Store), adds accountant-ready exports and advanced reports. Keel on the App Store.

Frequently asked questions

Do I have to lodge a BAS if I had no sales? Yes. If the ATO has issued you an activity statement for the period, you lodge it even when there is nothing to report, entering zero at each label. A nil BAS takes a few minutes online. Not lodging leaves the statement outstanding and can lead to penalties, so lodge it by the due date.

Can I do my own BAS, or do I need a BAS agent? You can do your own BAS through ATO online services or your accounting software; most sole traders with simple records manage it. If you pay someone to prepare or lodge it, they must be a registered BAS agent or tax agent, which you can check on the Tax Practitioners Board register. An agent may also have later lodgment dates.

How is BAS calculated? Add up what you owe: GST on sales at 1A, any PAYG instalment at 5A, and any tax withheld from wages. Subtract what you are owed: GST on purchases at 1B. A positive result is the amount to pay; a negative one is a refund. For GST, one-eleventh of a GST-inclusive amount is the GST inside it.

Does G1 include GST? Yes. G1 is total sales including GST, and the paper form asks you to confirm that it does. G1 also includes GST-free sales, which carry no GST, so 1A is not always exactly one-eleventh of G1. Work out 1A from your taxable sales only, then check the figures against your invoices and bank deposits before lodging.

What happens if I lodge or pay my BAS late? Late lodgment can attract a failure to lodge penalty, and unpaid amounts accrue the general interest charge until paid. If you cannot pay, lodge on time anyway and contact the ATO about a payment plan, which is treated very differently from silence. For the dates that apply to you, including agent concessions, see the BAS due dates guide.

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This article is general information, not tax advice. Consult a qualified Australian tax professional or registered BAS agent.