BAS 101: what a business activity statement is, when it is due, and what late costs

The business activity statement is the most regular piece of compliance a small business has. Most businesses lodge four a year, every year, forever - and a surprising number of owners have never been told what it actually reports or why the numbers land where they do.

This is the plain version.

What a BAS is

A BAS is a periodic report to the ATO covering the tax obligations that fall due through the year rather than at year end. Depending on your registrations, it can include:

• GST - what you collected on sales, less what you paid on purchases
• PAYG withholding - tax withheld from employees' wages
• PAYG instalments - prepayments towards your own income tax
• Other - fuel tax credits, luxury car tax, wine equalisation tax, and FBT instalments where they apply

A common misunderstanding worth clearing up: a BAS is not a tax return. It does not calculate your annual profit or your final tax bill. It reconciles amounts you have already collected or withheld on the ATO's behalf, plus instalments towards a liability that gets settled properly at year end.

You do not register "for BAS". You get BAS obligations as a consequence of registering for GST, or PAYG withholding, or entering the PAYG instalment system.

How the GST part works

GST is 10% on most goods and services sold in Australia. If you are registered, you collect it on your sales and claim back what you paid on your business purchases. The difference is what you remit.

The single most useful rule: to find the GST inside a GST-inclusive amount, divide by 11. Not by 10, and not 10% of the total - one-eleventh. A $110 invoice contains $10 of GST. Our GST calculator does it in both directions if you want to check one quickly.

Worked through a quarter:

• Sales of $1,100,000 including GST → GST collected is $1,100,000 ÷ 11 = $100,000
• GST-bearing expenses of $550,000 including GST → GST credits are $550,000 ÷ 11 = $50,000
• GST payable = $100,000 − $50,000 = $50,000

Note "GST-bearing" expenses. Wages carry no GST. Neither do most bank fees, most interest, superannuation, or purchases from suppliers who are not registered. Coding those as though they carried GST is one of the most common errors in a self-managed file, and it overstates your credits - which the ATO eventually notices.

Who has to register for GST

You must register once your GST turnover reaches $75,000 on a rolling twelve months - or $150,000 for a non-profit. Taxi, limousine and ride-sourcing drivers must register from their first fare regardless of turnover.

You have 21 days from crossing the threshold, and registration backdates to the day you crossed rather than the day you applied. We covered that in detail separately, because getting it wrong is expensive.

Registration is free through ATO online services, or through the Australian Business Register if you need an ABN at the same time. If you would rather not, we also run GST Register, which puts the application in front of a registered agent and turns most of them around in a couple of business hours - or your BAS or tax agent can simply handle it for you.

When a BAS is due

ATO quarters follow the financial year, not the calendar year - which trips people up constantly.

Quarter	Period	Due
1	July – September	28 October
2	October – December	28 February
3	January – March	28 April
4	April – June	28 July

The December quarter gets two months instead of one, in recognition of the holiday period.

Monthly lodgers - mandatory above $20 million turnover, optional below it - report by the 21st of the following month. January's BAS is due 21 February.

If you lodge through a registered agent, most quarters attract additional time under the agent lodgment program. The December quarter is the exception: its late-February date already reflects the concession. This is a real and often underrated benefit of using an agent - it is not just the work being done, it is more room to do it in.

Where a due date falls on a weekend or public holiday, it moves to the next business day.

What late lodgement costs

Failure to lodge on time attracts a penalty of one penalty unit for each 28 days (or part of 28 days) that the statement is overdue, capped at five units. From 1 July 2026 a Commonwealth penalty unit is $364.

The multiplier depends on entity size:

Entity	Turnover	Per 28 days	Maximum
Small	Under $1m	$364	$1,820
Medium	$1m – $20m	$728	$3,640
Large	$20m+	$1,820	$9,100

Note "per statement". Four overdue quarters is four penalties, not one.

The ATO generally does not apply the penalty where the lodgement is nil or results in a refund, and there is a safe harbour where a taxpayer engaged a registered BAS or tax agent, gave them what they needed, and the delay was the agent's. That exemption does not extend to statements lodged after a formal warning, or to taxable payments annual reports.

Separately, unpaid amounts accrue the general interest charge, compounding daily. For the July to September 2026 quarter the GIC annual rate is 11.43%, reset each quarter - and since 1 July 2025 it is no longer tax deductible, which makes the real cost considerably higher than the headline. We worked through what that actually means here.

The rule that saves the most money

Lodge on time even when you cannot pay.

Lodging and paying are separate obligations with separate consequences. Lodging on time and paying late gets you interest and a payment plan. Not lodging at all gets you interest, a failure-to-lodge penalty, and - for a company more than three months late on activity statements - a lockdown director penalty notice, which makes the director personally liable with no 21-day window to fix it.

A nil-payment BAS lodged on the due date costs nothing and preserves every option. It is the cheapest thing a struggling business can do, and it is routinely the thing that does not get done.

Getting it right without doing it yourself

Most BAS problems are not lodgement problems - they are coding problems that only become visible at lodgement. GST claimed on wages. Purchases from unregistered suppliers coded as though they carried GST. Bank feeds reconciled by guesswork.

Correct coding through the quarter makes the BAS a review rather than a reconstruction. That is the actual job, and it is what a registered BAS agent is for.

LINK Books is a registered BAS agent - our registration number and the Tax Practitioners Board register are on our TPB page. If your BAS is currently a stressful fortnight every quarter, or you are behind and need it untangled, catch-up bookkeeping is the service, and the first conversation costs nothing.

General information only, current as at the date above. Penalty units, interest rates and thresholds change - confirm with the ATO or your adviser before acting.