BAS Due Dates 2026-27 and How to Calculate GST for Each Quarter

Every guide to BAS deadlines tells you the same four dates: 28 October, 28 February, 28 April and 28 July. Three of them are right. The fourth is wrong for 2026-27, and the reason it is wrong is the same reason those four dates alone were never the whole answer — the deadline depends on how you lodge, and it moves when it lands on a day the ATO does not count as a business day.

Here is the real calendar, then the arithmetic.

The real 2026-27 quarterly BAS dates

Two things in that table are not in the usual list.

The December quarter is not due on 28 February

28 February 2027 falls on a Sunday. Where a lodgment or payment date is not a business day, the ATO lets you lodge and pay on the next business day — so the deadline moves to Monday 1 March.

Except Monday 1 March 2027 is Labour Day in Western Australia. The rule is broader than most people assume: where the date falls on a public holiday observed across the whole of any state or territory, taxpayers in every state and territory get the deferral, whether or not they observe that holiday. A business in Sydney gets the same extra day as one in Perth.

So the December-quarter BAS for 2026-27 is genuinely due Tuesday 2 March 2027 — two days later than the date on every calendar that just prints "28 February". If you have been treating the end of February as your hard stop, you have slightly more room than you thought. If you were planning to pay on 1 March because you knew about the Sunday, you were about to hit a day the ATO does not count either.

This is not a concession you apply for. It applies automatically.

The December quarter is also the one that gets no extra time

The second surprise in that table is that Q2's three columns are identical, while every other quarter's are not.

Lodging online yourself buys an extra two weeks. Lodging through a registered tax or BAS agent buys four. Neither applies to the December quarter, because its due date already carries a built-in one-month extension — the other three quarters are due 28 days after the period ends, while Q2 gets almost two months to cover the Christmas shutdown. The ATO does not extend an extension.

That has a practical consequence worth planning around. For Q1, Q3 and Q4 an agent gives you a month of breathing room. For Q2 your agent's deadline is your deadline, and it arrives in the same weeks as everything else that stacks up after the summer break.

Working out the GST for the quarter

You need two totals: GST-inclusive sales, and the GST-inclusive purchases you can actually claim credits on. That second one excludes GST-free purchases like basic food, anything private, and anything relating to input-taxed supplies such as residential rent.

The arithmetic is one operation in each direction:

• GST on sales = total GST-inclusive sales ÷ 11
• GST credits = total creditable GST-inclusive purchases ÷ 11
• Net GST = GST on sales − GST credits

Dividing by 11 rather than taking 10% is the part people get wrong. GST is 10% of the pre-GST price, so it is one-eleventh of the GST-inclusive total, not one-tenth. On a $110 sale the GST is $10, and $10 is 9.09% of $110.

A worked quarter:

• GST-inclusive sales $42,900 → GST collected = $3,900
• Creditable GST-inclusive purchases $16,500 → GST credits = $1,500
• Net GST payable: $2,400

And the direction people forget: if you bought equipment that quarter, the net figure can go the other way. Same $42,900 of sales, but $60,500 of creditable purchases gives $5,500 of credits against $3,900 collected — a $1,600 refund rather than a bill. A quarter with a big capital purchase in it is often a refund quarter.

Where the figures go on the form

Most small businesses report on Simpler BAS, which applies below $10 million of GST turnover and asks for only three GST figures:

• G1 — total sales, GST inclusive
• 1A — GST on sales, the figure you worked out above
• 1B — GST on purchases, your credits

Everything else on the classic form — the export, input-taxed and capital-purchase breakdowns — is not required at that turnover. If you are staring at a long list of labels and running a small business, you are probably looking at the wrong form.

The GST you collect was never your money

The single most useful habit in GST is treating collected GST as held, not earned. Every GST-inclusive sale you bank contains roughly 9% that belongs to the ATO, and a quarterly cycle means you can be holding up to four months of it before the bill lands.

Businesses that come unstuck on BAS payments usually did not miscalculate anything. They spent working capital that was never theirs, then met a deadline they could not fund. Moving one-eleventh of each deposit into a separate account on the day it arrives costs nothing and removes the problem entirely.

Monthly or quarterly

Quarterly is the default for small business. You must report monthly once GST turnover reaches $20 million, and you can elect monthly voluntarily — some businesses do, precisely to avoid holding a large balance for months.

Below $75,000 of GST turnover ($150,000 for not-for-profits) you do not need to register for GST or lodge a BAS at all. Registering voluntarily below the threshold lets you claim GST credits on purchases, at the cost of charging GST on sales and lodging every quarter.

What lodging late costs

The failure-to-lodge penalty runs at one penalty unit for each 28 days, or part of a period, that the statement is overdue — capped at five units for a small entity. A penalty unit has been $364 since 1 July 2026, up from $330, so the maximum exposure on one late BAS is $1,820.

Note what triggers it: lodging late, not paying late. Late payment is a separate matter that attracts interest rather than this penalty. If cash is the problem and the paperwork is not, lodge on time anyway and arrange a payment plan — it is the cheaper of the two failures by a wide margin.