If you run a business in Australia, you will encounter either a BAS, an IAS, or in some cases both. The two forms look similar, have overlapping due dates, and report some of the same obligations. No surprise that most small business owners either confuse them or do not realise they are separate documents with different purposes.
Getting this wrong can mean lodging the wrong form, missing a lodgement entirely, or failing to report an obligation that was due on one form rather than the other. All of these attract ATO penalties.
This guide explains exactly what each form does, who needs to lodge which one, and the specific situations where you need both.
A Business Activity Statement is a form lodged with the Australian Taxation Office that reports multiple tax obligations in a single document. The BAS is the primary reporting form for businesses registered for Goods and Services Tax (GST).
A BAS can include GST collected on sales and GST paid on purchases (the net amount determines whether you owe the ATO or receive a refund), PAYG withholding (tax deducted from employee wages and remitted to the ATO), PAYG instalments (quarterly prepayments of your own business income tax), Fringe Benefits Tax (FBT) instalments, fuel tax credits, wine equalisation tax, and luxury car tax.
Not every BAS includes all of these. A sole trader with no employees might only report GST. A company with 20 employees might report GST, PAYG withholding, and PAYG instalments. The sections you need to complete depend on your registered obligations.
You must lodge a BAS if you are registered for GST. GST registration is mandatory when your annual turnover reaches $75,000 ($150,000 for non-profits). You can also register voluntarily below these thresholds. For a full breakdown of BAS preparation and lodgement, see our guide to preparing and lodging a BAS.
An Instalment Activity Statement is a form lodged with the ATO that reports a subset of the obligations that might otherwise appear on a BAS. Specifically, the IAS covers PAYG withholding, PAYG instalments, and FBT instalments.
The key difference is that the IAS does not include GST. It exists for entities that have tax reporting obligations but are not registered for GST.
The ATO sends you an IAS if you are registered for PAYG withholding or PAYG instalments but are not registered for GST, or if you are a medium withholder who lodges quarterly BAS for GST but must report PAYG withholding monthly (in which case, you lodge IAS for the non-quarter-end months).
The simplest way to think about it is this. If you are registered for GST, you lodge a BAS. Your PAYG withholding, PAYG instalments, and other obligations are reported on that same BAS alongside your GST figures.
If you are not registered for GST but have PAYG withholding or instalment obligations, you lodge an IAS instead.
The IAS is essentially a BAS without the GST section. The PAYG reporting labels are the same (W1, W2 for withholding; T1, T2 for instalments). The due dates are the same. The penalties for late lodgement are the same. The only structural difference is the presence or absence of GST reporting.
Here is how the two forms compare across the key dimensions.
On the question of who lodges: BAS is for businesses registered for GST (mandatory over $75,000 turnover). IAS is for businesses not registered for GST but with PAYG withholding, PAYG instalment, or FBT instalment obligations.
On the question of what it reports: BAS reports GST, PAYG withholding, PAYG instalments, FBT instalments, fuel tax credits, wine equalisation tax, and luxury car tax. IAS reports PAYG withholding, PAYG instalments, and FBT instalments only. No GST.
On the question of reporting frequency: BAS is lodged monthly (turnover above $20 million), quarterly (most SMEs), or annually (voluntary registrants under $75,000). IAS is lodged monthly or quarterly depending on your withholder category and instalment obligations.
On the question of due dates: Quarterly BAS is due 28 days after the end of each quarter (28 October, 28 February, 28 April, 28 July). Monthly BAS is due on the 21st of the following month. IAS follows exactly the same schedule. Quarterly IAS shares the same dates as quarterly BAS. Monthly IAS is due on the 21st of the following month.
On the question of penalties: Both attract the same Failure to Lodge penalty of $330 per penalty unit (one unit per 28-day period overdue, up to five units). Both attract General Interest Charge on late payments. Both can trigger Director Penalty Notices for unpaid PAYG withholding.
On the question of lodgement method: Both can be lodged through the ATO Business Portal, accounting software (Xero, MYOB), or a registered BAS agent.
There is one specific scenario where a business lodges both forms. This applies to medium withholders who are also registered for GST on a quarterly basis.
A medium withholder is a business that withholds between $25,001 and $1 million in PAYG per year. Medium withholders must report and pay PAYG withholding monthly, regardless of their GST reporting frequency.
If the business lodges quarterly BAS for GST, the quarter-end month's PAYG withholding is included on the BAS. But the two non-quarter-end months need a separate form, which is the IAS.
Here is a worked example. A Sydney marketing agency has 12 employees with a combined annual payroll of $950,000. Annual PAYG withholding is approximately $200,000, making the business a medium withholder. The agency is GST-registered and lodges quarterly BAS.
For the October to December quarter, the agency lodges a monthly IAS for October (due 21 November) reporting that month's PAYG withholding of approximately $16,700. It lodges another monthly IAS for November (due 21 December) reporting another $16,700. Then for December, the PAYG withholding is included on the quarterly BAS (due 28 February), along with the quarter's GST figures.
So in a typical quarter, this business lodges two IAS forms and one BAS. Over a full year, that is eight monthly IAS forms plus four quarterly BAS forms, for a total of 12 lodgements. Missing any single one triggers an FTL penalty.
To make this concrete, here are five common business scenarios.
Scenario 1: A sole trader graphic designer earning $120,000 per year with no employees. GST-registered (turnover above $75,000). This business lodges a quarterly BAS reporting GST only. No IAS needed. No PAYG withholding because there are no employees. PAYG instalments may appear on the BAS if the ATO has assessed them.
Scenario 2: A small retail shop with two casual employees and $60,000 annual turnover. Not GST-registered (below $75,000 threshold). This business lodges a quarterly IAS reporting PAYG withholding from the two employees. No BAS because not registered for GST.
Scenario 3: A tech startup with 6 employees, $2 million revenue, GST-registered. Annual PAYG withholding is approximately $120,000 (medium withholder). This business lodges monthly IAS for non-quarter-end months (PAYG withholding only) and quarterly BAS (GST plus that month's PAYG withholding plus PAYG instalments if applicable). Total: 8 IAS plus 4 BAS per year.
Scenario 4: A property investor with no employees and $40,000 in rental income. Not GST-registered (residential rent is input-taxed, not subject to GST). The ATO may issue PAYG instalment notices based on the investment income. This person lodges a quarterly IAS reporting PAYG instalments only. No BAS, no PAYG withholding.
Scenario 5: A trades business with 15 employees, $4 million revenue, GST-registered. Annual PAYG withholding is $280,000 (medium withholder). Same structure as Scenario 3. Monthly IAS for non-quarter-end months, quarterly BAS for everything else. If annual withholding exceeded $1 million (which it does not in this case), the business would be a large withholder with different payment timing, but the BAS/IAS structure would remain similar.
If you are GST-registered and accidentally lodge an IAS instead of a BAS (or vice versa), the ATO may reject the lodgement or contact you to correct it. In the meantime, the clock is ticking on your lodgement deadline, and an FTL penalty can apply from the original due date if the correct form is not lodged in time.
If you receive an IAS from the ATO but believe you should be lodging a BAS (or the reverse), check your registration details on the ATO Business Portal. You may need to update your GST registration status.
The more common error is simply not realising you need to lodge at all. This happens frequently when a business is not GST-registered but has employees. The business owner focuses on running payroll and STP reporting but does not realise they also need to lodge an IAS to report and pay the withheld amounts. STP reporting alone does not satisfy the lodgement obligation. The IAS (or BAS) is the payment and reconciliation form.
If your business grows and you become required to register for GST (because your turnover hits or is expected to hit $75,000), you transition from lodging IAS to lodging BAS. The BAS will then capture your GST obligations alongside your existing PAYG withholding and instalment obligations.
Conversely, if your turnover drops below $75,000 and you cancel your GST registration, you may transition from BAS to IAS (assuming you still have PAYG obligations from employees).
When you register or deregister for GST, the ATO automatically updates which form you receive. However, there can be a transitional period where the timing overlaps, and you need to be careful not to double-report or miss a period. Your BAS agent can manage this transition for you.
Scale Suite handles all BAS and IAS preparation and lodgement as part of our embedded finance service. We track your withholder category, manage the monthly/quarterly lodgement rhythm, reconcile PAYG withholding against STP data, and ensure every form is lodged on time. As a registered BAS agent (registration #26346881), we also access extended lodgement dates where applicable. If you are unsure whether you should be lodging a BAS, an IAS, or both, we can review your situation and get your reporting structure right.
Do IAS and BAS have the same due dates?
Yes. Quarterly IAS and quarterly BAS share the same due dates: 28 October, 28 February, 28 April, and 28 July. Monthly IAS and monthly BAS are both due on the 21st of the following month. Registered agent concessions apply to both.
Can I lodge a BAS if I am not registered for GST?
No. If you are not registered for GST, the ATO will not issue you a BAS. You lodge an IAS instead for your PAYG obligations.
What if I have no employees and no PAYG obligations?
If you are registered for GST, you still lodge a BAS (reporting GST only). If you are not registered for GST and have no PAYG obligations, you generally do not need to lodge either form. However, if the ATO has entered you into the PAYG instalment system based on your prior year tax return, you will receive IAS notices for those instalments.
Do I report super on a BAS or IAS?
Neither. Superannuation guarantee contributions are not reported on the BAS or IAS. Super is reported through STP and paid directly to each employee's super fund. The Superannuation Guarantee Charge (which applies when super is paid late) is a separate lodgement with the ATO.
What is the penalty for missing an IAS lodgement?
The same as missing a BAS lodgement. The FTL penalty is $330 per penalty unit, one unit per 28 days overdue, up to five units ($1,650 maximum for small entities). For medium entities, the penalty doubles. Late payment also attracts General Interest Charge at approximately 11% per annum.
Can my accountant lodge both my BAS and IAS?
Yes, provided they are a registered tax agent or BAS agent. Using a registered agent may also give you access to extended lodgement dates. Make sure your agent is aware of all your reporting obligations, especially if you are a medium withholder who needs both monthly IAS and quarterly BAS.
Disclaimer: We review and check articles periodically. At the time of writing, this information was up to date from our assessment. Tax obligations, thresholds, and penalty amounts change regularly. Always verify current requirements with the ATO or a registered tax professional for advice specific to your circumstances.
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