Q1 BAS for Construction and Solar Businesses: 7 Things to Check Before 28 October 2026

The July to September quarterly BAS is due on 28 October 2026 if you lodge it yourself. It's the first BAS of the new financial year, and for construction, EPC and solar businesses it's often the messiest one. Progress claims straddle quarters, retentions get forgotten, and fuel tax credit rates changed partway through the quarter.

Here are the seven things we check on every Q1 BAS for project-based clients.

1. Know your real due date

If you lodge yourself, it's 28 October. If you lodge online you may get an extra two weeks, and if you use a registered tax or BAS agent you may get longer again. Check with your agent rather than assuming.

Paying late now costs more than it used to. Since 1 July 2025, the general interest charge the ATO applies to overdue amounts is no longer tax deductible. If cash is tight, it's usually better to lodge on time and set up a payment plan than to lodge late.

2. Match GST on progress claims to the right quarter

If you account for GST on an accruals basis, GST on a sale belongs in the period you issue the tax invoice or receive any payment, whichever happens first. It doesn't matter when the work was done.

That means a progress claim invoiced on 29 September belongs in this BAS, even if the client won't pay until November. Pull a list of every claim invoiced between 1 July and 30 September and tie it to the GST at label 1A.

3. Check how your invoices treat retentions

Retentions are where we see the most GST errors. If your invoice shows the full claim amount and the client simply holds back the retention, GST is generally payable on the full amount now, not when the retention is released. If your invoices are set up differently, the answer can change.

Look at how your last few claims were worded and make sure your accounting system is treating retentions the same way. Our guide to GST on variations and retentions walks through the common traps.

4. Pick up approved variations

Variations get approved on site, priced in an email and then sit in someone's inbox. If a variation was invoiced in the quarter, the GST goes in this BAS. If it was approved but never invoiced, that's a billing problem, not a BAS problem, and worth chasing now.

5. Split your fuel tax credits at 3 August

Fuel tax credit rates changed on 3 August 2026. Fuel bought from 1 July to 2 August uses the old rates, and fuel bought from 3 August uses the new ones. The rates are also different for heavy vehicles on public roads and for off-road plant like excavators, generators and loaders.

If you claim $10,000 or more in fuel tax credits a year, claiming the whole quarter at one rate is one of the easiest ways to get the number wrong. If you claim less than that, the ATO lets you use the rate that applies on the last day of the BAS period. Either way, keep the fuel card statements that back up your litres. Our guide to fuel tax credits for construction plant has the 2026 rates and the records to keep.

6. Reconcile PAYG withholding to Single Touch Payroll

The wages and tax withheld on your BAS (labels W1 and W2) should match what your payroll software reported through Single Touch Payroll for the same three months. Differences usually come from back pay, termination payments or a pay run processed twice.

While you're in payroll, check your super. Since payday super started on 1 July 2026, super has to reach employees' funds within seven business days of each payday instead of each quarter. If any Q1 contributions went in late, deal with them now rather than at year end. Our payday super guide covers what changed.

7. Decide whether your PAYG instalment still makes sense

Your Q1 PAYG instalment is based on last year's income. If this year is shaping up very differently, because a big project finished or a contract fell over, you can vary the instalment. Vary too low and you may pay interest, so base it on a proper forecast, not a guess. We cover how to do that in PAYG instalments and lumpy project cash flow.

One more for builders who issue RCTIs

If you issue recipient created tax invoices to subcontractors, make sure each RCTI issued in the quarter is in your GST credits, and that you have a current written RCTI agreement with every subbie. Our RCTI walkthrough has the detail.

Need a hand?

TechEdge is a CPA-led outsourced finance team for Australian construction, EPC, solar and renewables businesses. We prepare and review BAS as part of our Financial Controller and Head of Finance engagements, with the job costing behind it done properly.

If your Q1 BAS is looking messy, email info@techedgeaccounting.com.au or call +61 468 634 973.

This article is general information only and is not financial, tax or legal advice. Dates and rules reflect the position as at October 2026; check the ATO website or speak to your adviser about your own circumstances.