September monthly BAS due 21 October: a short checklist

Who lodges monthly BAS?

Most small businesses lodge quarterly, but monthly BAS is common when turnover is over $20 million, or when a business chooses monthly reporting to improve cash-flow visibility and stay closer to its numbers. Some businesses also switch to monthly at the request of the ATO.

The trade-off is more frequent deadlines. Instead of four quarterly lodgements, you have twelve monthly ones, with each month's BAS due on the 21st of the following month. September's activity statement therefore lands on 21 October — the same month as the quarterly rush.

What to reconcile before 21 October

Because the reporting window is only one month, September's transactions need to be clean and complete. Set aside time in the first week of October for these checks:

• Reconcile every bank account and credit card for September — unreconciled items are the biggest source of BAS errors.
• Code all GST receipts and invoices correctly in QBO, Xero or MYOB before running the BAS report.
• Check that customer invoices issued in September have been paid or recorded accurately; write off bad debts only when the rules allow.
• Confirm supplier bills are entered and GST claimed only where you are entitled to it.
• If you employ staff, make sure every September pay run is posted and reported through STP.
• Review any private or non-business transactions so they are not accidentally included in GST claims.

Common monthly BAS mistakes

The same errors show up every month, and they are all avoidable with a short checklist:

• Claiming GST on expenses that are GST-free or input-taxed, such as some bank fees, residential rent or entertainment.
• Forgetting to report GST on cash vs accrual basis consistently — your accounting method must match what is registered with the ATO.
• Missing fuel tax credits or other offsets you are entitled to claim.
• Leaving unreconciled bank items sitting in suspense instead of coding them properly.
• Lodging late because the deadline was mistaken for the quarterly 28 October date.

Why a registered BAS agent helps

A registered BAS agent can prepare and lodge activity statements on your behalf, and may have access to extended lodgement dates through the Tax Practitioners Board regime. More importantly, they spot problems before the BAS is lodged — incorrect GST claims, missing income, and coding errors that can trigger ATO questions later.

For monthly lodgers, the faster rhythm means small issues compound quickly. Having a bookkeeper or BAS agent check September's figures in early October keeps the 21st deadline calm and your cash-flow picture accurate.

What if September is already messy?

If September's bank feed is half-coded and receipts are scattered, don't wait until 20 October. Catch-up bookkeeping works best when it starts early, while transactions are still fresh and source documents are still available.

We regularly take businesses from a month of chaos to a clean BAS in a few focused sessions. The earlier you start, the more options you have — including lodging through a BAS agent if an extended deadline is available.