A Business Activity Statement (BAS) is the ATO form GST-registered businesses use to report GST and often PAYG withholding, PAYG instalments, and other obligations. An Instalment Activity Statement (IAS) is for reporting PAYG instalments and/or PAYG withholding without GST — typically when you are not registered for GST, or when you need a monthly PAYG statement in months you are not lodging a quarterly BAS. Which one you lodge depends on your GST registration and the reporting cycle the ATO has set for you.
That is the short answer. Below is a plain-English guide for Australian sole traders, companies, and small employers who keep seeing both acronyms and want to know which form is theirs — without rewriting a full BAS how-to or due-date calendar.
This is bookkeeping and activity-statement guidance, not income-tax strategy. For tax returns, structuring, Division 7A, or deduction planning, speak to your registered tax agent or accountant. Girl Friday Australia is a registered BAS agent, Xero Certified Advisor, and Xero Gold Partner — we prepare and lodge BAS & IAS, keep your books tidy, and hand tax strategy to your tax agent when needed.
What is a BAS?
BAS stands for Business Activity Statement. According to the Australian Taxation Office (ATO), if you are a business registered for GST, you generally need to lodge a BAS. Your BAS helps you report and pay:
- Goods and services tax (GST)
- Pay as you go (PAYG) instalments (where applicable)
- PAYG withholding (tax withheld from employees or other payments)
- Other amounts that apply to you — for example FBT instalments, luxury car tax, wine equalisation tax, or fuel tax credits
Most Australian small businesses with GST turnover under $20 million report GST quarterly. Larger GST turnovers (or ATO direction / your choice) can mean monthly BAS lodgement. Some voluntary GST registrants under the threshold may report annually. Always check what appears on your activity statement and in Online services — the ATO sets your cycle.
For a deeper BAS-and-books map (not this comparison), see our BAS, bookkeeping & tax guide. For quarter due dates, use BAS due dates 2026–27 rather than guessing from memory.
What is an IAS?
IAS stands for Instalment Activity Statement. It is an activity statement used when you need to report and/or pay PAYG instalments and/or PAYG withholding — but not GST on that form. The ATO issues different IAS layouts depending on your obligations (for example PAYG instalments only, or PAYG withholding only).
You typically see an IAS when:
- You are not registered for GST, but you have PAYG instalment obligations (common for some sole traders, partners, investors, or SMSF trustees with instalment income).
- You withhold PAYG from employees or other payments but you are not GST-registered, so there is no GST to report on a BAS.
- You lodge a quarterly BAS, but you also have monthly PAYG withholding (or monthly PAYG instalments) — so the ATO issues monthly activity statements for the first two months of the quarter, then a quarterly BAS for the third month.
- You have another instalment obligation (such as an FBT instalment) that the ATO is collecting through an instalment activity statement rather than a GST BAS.
Quick visual check: if your form has GST labels such as G1 / 1A / 1B, you are looking at a BAS. If you mainly see withholding (W) and instalment (T) labels without GST sales fields, you are usually looking at an IAS.
IAS vs BAS: side-by-side comparison
Use this as a orientation guide — your exact labels and due date are always on the statement the ATO issues you.
What each form usually covers
- BAS — GST (core), plus PAYG withholding, PAYG instalments, and other taxes/credits that apply (FBT instalments, LCT, WET, fuel tax credits where relevant).
- IAS — PAYG instalments and/or PAYG withholding (and sometimes FBT instalments). No GST reporting on the IAS.
Who typically lodges
- BAS — GST-registered businesses (mandatory GST registration generally from $75,000 GST turnover for most businesses; higher threshold for many not-for-profits).
- IAS — Taxpayers who need to report PAYG without GST on that statement, or quarterly BAS lodgers who also have monthly PAYG cycles in the “off” months.
Typical frequency
- BAS — Quarterly for most SMEs; monthly if GST turnover is $20 million or more (or you are told / choose monthly); annually in limited voluntary-GST cases.
- IAS — Monthly or quarterly depending on your PAYG obligations. The ATO sets this from your registration profile — you lodge what they issue.
Due dates (high level only)
- Monthly activity statements are generally due on the 21st of the following month (with limited December concessions for some monthly GST lodgers — check ATO rules for your case).
- Quarterly BAS legislative due dates are commonly 28 October, 28 February, 28 April, and 28 July for the four standard quarters. Electronic lodgement and registered-agent lodgment-program concessions can change your practical due date — see the ATO and our BAS due dates 2026–27 post rather than relying on a blog calendar alone.
Source pages to bookmark: ATO — Business activity statements and ATO — Activity statement due dates (agents).
When you lodge an IAS vs a BAS
You do not get to pick the form for fun. The ATO generates activity statements from your registrations (GST, PAYG withholding, PAYG instalments, and so on).
Lodge a BAS when:
- You are registered for GST and the ATO has issued a BAS for the period.
- Your statement includes GST labels — even if other PAYG amounts also appear on the same BAS.
- You are on a monthly, quarterly, or annual GST reporting cycle and that period’s BAS is available in Online services or with your agent.
Lodge an IAS when:
- The ATO has issued an Instalment Activity Statement for PAYG instalments and/or PAYG withholding without GST on that document.
- You are a quarterly GST lodger with monthly PAYG withholding: you will usually receive monthly statements for the first two months of the quarter, then a quarterly BAS that covers the third month (and your GST for the quarter).
- You are not GST-registered but still have instalment or withholding reporting to complete.
Important: a nil statement still usually needs to be lodged. “Nothing to report” is not the same as “ignore the form.” Failure to lodge can attract penalties on activity statements whether they are a BAS or an IAS.
How GST registration and PAYG affect which form you get
Think of three switches the ATO looks at:
- GST registration — On? Expect a BAS (with GST) for your GST reporting periods.
- PAYG withholding — Do you withhold from wages or other payments? You must report that withholding on the activity statement cycle the ATO sets (monthly or quarterly). That reporting can sit on a BAS or on an IAS.
- PAYG instalments — Has the ATO put you into PAYG instalments on your business or investment income? You report and/or pay those instalments through an activity statement or instalment notice.
Common combinations
- GST + quarterly BAS only — Many sole traders and small companies without employees. One BAS each quarter covering GST (and PAYG instalments if you are in the system).
- GST + employees with monthly withholding — Quarterly BAS for GST, plus monthly activity statements (often IAS-style) in the two non-quarter-end months for PAYG withholding.
- No GST, but employees — IAS (or similar activity statement) for PAYG withholding; no GST BAS.
- No GST, PAYG instalments only — IAS or a PAYG instalment notice. Sometimes you only need to pay the notice amount unless you are varying it — follow the instructions on the notice and ATO guidance.
If your software only knows about “the BAS,” monthly IAS periods are easy to miss. That is one of the most common mix-ups we clean up for clients who hire staff mid-year or cross the withholding threshold.
Common mix-ups for sole traders and companies
“I got an IAS this month and a BAS last month — is the ATO confused?”
Usually not. If you are on quarterly GST and monthly PAYG withholding, alternating forms across a quarter is normal. Diarise both due dates — they are often different (21st vs 28th patterns).
“Can I put everything on the BAS and skip the IAS?”
No. Lodge the form the ATO issued for that period. Skipping an IAS because it “isn’t a real BAS” is a fast path to failure-to-lodge risk.
“My company is GST-registered — so I never see an IAS.”
Not always true. Monthly PAYG withholding alongside quarterly GST is exactly when many companies still see IAS (or monthly activity statements) in the first two months of each quarter.
“I’m a sole trader under the GST threshold — why do I have anything?”
You might still have PAYG instalments after a prior tax assessment, or PAYG withholding if you have staff or certain payments. GST registration is only one switch.
“Nil IAS — do I still lodge?”
Yes in almost all cases. Confirm on the statement or with your registered BAS agent, but treating nil as optional is a common (and costly) myth.
“Xero already lodged my BAS — I’m done for the quarter.”
Check whether a separate monthly statement is still sitting in Online services. Software workflows need to match your full activity-statement profile, not just the quarterly GST BAS. For a clean Xero BAS process once the right periods are in view, see How to prepare and lodge your BAS in Xero.
How a registered BAS agent helps (plain English)
A registered BAS agent can:
- Read your ATO activity-statement profile and explain whether each period is a BAS or an IAS.
- Prepare figures from reconciled books (GST coding, wages withholding, instalment income where relevant).
- Lodge electronically through agent channels and, where the lodgment program applies, help you use available due-date concessions for eligible quarterly statements.
- Catch the “two monthly + one quarterly” rhythm so PAYG withholding is not double-counted or skipped.
- Keep bookkeeping and lodgement in one rhythm so you are not scrambling every 21st and 28th.
What we don’t do: income-tax return strategy, trust/company structuring advice, or “how much tax will I owe at EOFY” forecasting beyond BAS-agent scope. That is your tax agent’s lane — and that split is a feature, not a bug.
Girl Friday’s BAS & IAS lodgement starts from $180/quarter, and ongoing bookkeeping from $250/month, scoped to your volume. See BAS & IAS services and tax support for how we fit beside your accountant.
Not sure which statement you should be lodging?
Send us a screenshot of your upcoming obligations (or just tell us your GST and payroll setup). We will tell you in plain English whether you are on a BAS cycle, an IAS cycle, or both — then quote the cleanest way to stay on top of it.
Get a free quote · Contact Girl Friday · Call 0404 877 436 · Email info@girlfridayaustralia.com
Frequently asked questions
Is an IAS the same as a BAS?
No. A BAS includes GST reporting for GST-registered businesses (and often other obligations on the same form). An IAS is used for PAYG instalments and/or PAYG withholding without GST on that statement. Same ATO activity-statement family — different jobs.
Can I lodge an IAS if I am GST-registered?
Yes, in the common case where you have monthly PAYG obligations and quarterly GST. You still lodge your quarterly BAS for GST; the IAS (or monthly activity statement) covers the intervening monthly PAYG periods. You do not replace the BAS with an IAS for GST.
Do sole traders lodge BAS or IAS?
It depends. GST-registered sole traders lodge a BAS. Sole traders who are not GST-registered may still lodge an IAS for PAYG instalments or withholding. Check Online services for individuals and sole traders or ask your registered BAS agent.
What if I miss an IAS?
Treat it as seriously as a missed BAS. Failure-to-lodge penalties and interest on unpaid amounts can apply. Lodge as soon as you can, then speak to the ATO (or your agent) about payment plans or penalty remission if needed — remission is never guaranteed.
Does a registered BAS agent lodge both BAS and IAS?
Yes. Preparing and lodging activity statements (including BAS and IAS) is core registered BAS agent work. Girl Friday lodges both as part of BAS & IAS services from $180/quarter, alongside bookkeeping when you want the books and lodgements handled together.
Where do I see which form is due?
Log in to ATO Online services (business or individuals/sole traders, as applicable) or ask your agent to check Online services for agents. The upcoming lodgments list shows the form type and due date for your entity — that beats guessing from a generic calendar.
Is this tax advice?
No. This article is general information about activity statements for Australian small businesses. It is not personal tax advice. For income-tax strategy, speak to a registered tax agent or accountant.
Get BAS & IAS sorted without the jargon
If IAS vs BAS still feels like alphabet soup, you do not need to memorise every ATO form code. You need accurate books, the right statement lodged on time, and a registered BAS agent who will tell you which is which in one sentence.
Girl Friday Australia — registered BAS agent, Xero Certified Advisor, Xero Gold Partner — helps Australian small businesses with bookkeeping from $250/month and BAS & IAS from $180/quarter. Australia-wide remote. No lock-in contracts.
- Get a free quote
- Book a discovery call / contact us
- BAS & IAS services
- Phone 0404 877 436
- Email info@girlfridayaustralia.com
Mon–Fri 9am–5pm AEST. WhatsApp welcome: https://wa.me/61404877436