When are BAS and IAS due in Australia?

January 6, 2026
Tax & Compliance
If you run a business or employ staff in Australia, BAS and IAS deadlines are part of life. Even so, they are often a source of stress and confusion. Many people are unsure which form applies to them, when it is due, and what happens if they miss the deadline.
As accountants working with businesses across Port Stephens and Nelson Bay, we see the same questions come up every year. Some clients lodge on time without issue. Others only realise a due date has passed when an ATO notice arrives.
This guide explains BAS and IAS due dates in plain language. It also explains how the due dates change depending on how often you lodge, whether you use a registered agent, and what practical steps can make compliance easier.

What is a BAS?

A Business Activity Statement (BAS) is used to report and pay certain taxes to the ATO. Most commonly, this includes:
  • GST collected and paid
  • PAYG withholding from employees
  • PAYG instalments for income tax
  • Other taxes, such as the wine equalisation tax, if applicable
If your business is registered for GST, you will usually need to lodge a BAS.

What is an IAS?

An Instalment Activity Statement, or IAS, is used when you are not registered for GST but still have tax obligations to report. This often includes:
  • PAYG withholding
  • PAYG instalments
IAS lodgers are often sole traders, investors, or businesses below the GST registration threshold.
Understanding whether you lodge a BAS or an IAS is important because the forms and due dates can differ.

How often is BAS lodged?

The ATO sets your BAS lodgement frequency based on your business size and turnover. The most common options are quarterly and monthly.

Quarterly BAS

Most small businesses lodge BAS quarterly. This is the most common setup for small business accounting in Port Stephens.
Quarterly BAS periods are:
  • July to September
  • October to December
  • January to March
  • April to June

Quarterly BAS due dates

For businesses lodging their own BAS:
  • The July to September quarter is due 28 October.
  • The October to December quarter is due 28 February.
  • The January to March quarter is due on 28 April.
  • The April to June quarter is due on 28 July.
There is one important exception. If you lodge the December quarter yourself, the due date is still 28 February. This is later than the usual one-month window and often catches people off guard.

Monthly BAS

Some businesses are required to lodge BAS monthly. This is more common for larger businesses or those with higher turnover.
Monthly BAS is due on the 21st day of the following month. For example:
  • July BAS is due on 21 August.
  • August BAS is due 21 September.
Monthly lodgement requires more frequent reporting, but it can help with cash flow management for some businesses.

IAS due dates

IAS is generally lodged monthly or quarterly, depending on your situation.

Monthly IAS

If you are required to lodge IAS monthly, it is due on the 21st of the following month, similar to monthly BAS.

Quarterly IAS

Quarterly IAS due dates are the same as quarterly BAS dates:
  • 28 October
  • 28 February
  • 28 April
  • 28 July
Even though the form is different, the deadlines align.

What changes if you use a registered tax or BAS agent?

This is where many businesses gain breathing room.
If you use a registered tax agent or BAS agent, you may be eligible for extended due dates. This is one of the key benefits of working with a Nelson Bay tax agent or local accountants in Port Stephens.
For quarterly BAS clients on an agent lodgement program:
  • The September quarter BAS may be due in late November.
  • The December quarter BAS may be due in late December.
  • The March quarter BAS may be due in late May.
  • The June quarter BAS may be due in late August.
Exact dates depend on your agent’s lodgement schedule and whether prior lodgements are up to date.
It is important to note that these extensions apply to lodgement, not payment, in all cases. Your accountant will clarify what applies to you.

Why BAS and IAS deadlines matter

Missing a BAS or IAS due date can lead to penalties and interest. Even if you cannot pay the full amount, on-time lodging is still important.
The ATO is generally more flexible when you lodge on time and communicate early. Late lodgement without contact can lead to unnecessary stress.
Regular BAS lodgement help is not just about ticking a box. It helps you:
  • Stay aware of cash flow.
  • Avoid compounding debt
  • Keep ATO communication manageable.
  • Maintain eligibility for payment plans.

Common mistakes we see

After years of supporting bookkeeping services in Nelson Bay and BAS lodgement help, these are some of the most common issues we see:
  • Assuming BAS is not required because there was no activity
  • Forgetting the December quarter due date is different.
  • Mixing up BAS and IAS obligations
  • Lodging late because the records are not ready
  • Ignoring reminders because cash flow is tight
Even if your BAS shows zero activity, it still needs to be lodged.

How bookkeeping affects BAS and IAS

Accurate bookkeeping makes BAS and IAS easier and less stressful. When records are kept up to date, lodgement becomes a review process rather than a scramble.
Bookkeeping services in Nelson Bay often support businesses by:
  • Reconciling accounts regularly
  • Ensuring GST is coded correctly
  • Preparing reports ahead of due dates
  • Flagging issues early
This allows your accountant to focus on accuracy and advice rather than corrections.

BAS and cash flow planning

BAS obligations can put pressure on cash flow, especially for growing businesses. Regular discussions with your accountant help manage this.
Business advisory in Nelson Bay often includes:
  • Setting aside GST and PAYG amounts
  • Reviewing instalment rates
  • Adjusting PAYG instalments if income changes
  • Planning payment arrangements when needed
This turns BAS from a shock into something predictable.

What about SMSFs?

SMSF accounting in Port Stephens involves different reporting obligations, but trustees often confuse SMSF deadlines with BAS and IAS. SMSFs do not lodge BAS unless they are registered for GST.
If your SMSF is registered for GST, BAS obligations apply. Otherwise, SMSF reporting is handled through the annual return.
This is another area where clear advice matters.

How often should you review your BAS obligations?

At least once a year, it is worth reviewing:
  • Whether your lodgement frequency is still appropriate
  • Whether GST registration still makes sense
  • Whether PAYG instalments reflect your income
As businesses change, so do reporting needs.

Frequently Asked Questions

When is BAS due for small businesses?
Most small businesses lodge BAS quarterly. The due dates are 28 October, 28 February, 28 April, and 28 July.
Is the December BAS due later?
Yes. The October to December quarter BAS is due 28 February for self-lodgers.
What happens if I lodge late?
You may incur penalties and interest. Lodging on time, even without payment, is always better.
Does using an accountant extend BAS deadlines?
In many cases, yes. Registered agents often have extended lodgement dates.
What is the difference between BAS and IAS?
BAS is used for GST-registered businesses. IAS is used when GST is not required, but PAYG obligations exist.
Do I need to lodge if there is no activity?
Yes. A BAS or IAS that is zero still needs to be lodged.
Can a bookkeeper lodge my BAS?
A registered BAS agent can lodge a BAS. Many bookkeepers work alongside accountants to prepare the information.
How can I avoid BAS stress?
Regular bookkeeping, early preparation and working with expert accountants in Port Stephens and Nelson Bay make a big difference.

Final thoughts

BAS and IAS deadlines are not just administrative dates. They affect cash flow, compliance and peace of mind. Knowing when forms are due and having systems in place to meet those deadlines reduces unnecessary pressure.
Whether you lodge monthly or quarterly, on your own or through an agent, the goal is the same. Stay informed, stay organised and ask for help early if something feels unclear.
For many businesses, working with local accountants in Port Stephens turns BAS and IAS from a source of anxiety into a routine part of running a healthy business.
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