How inventory, COGS and stock write-offs flow to your BAS in Australia

If you sell physical goods in Australia, your stock quietly drives three different reporting jobs at once: the GST you report on your BAS, the cost of goods sold (COGS) that shapes your income tax, and the closing stock value that lands on your balance sheet. They use the same underlying movements, but they are not the same number, and confusing them is how a tidy-looking set of books produces a BAS that will not reconcile.

This guide walks through how inventory, COGS and obsolete-stock write-offs flow into your Business Activity Statement in Xero or MYOB, what belongs on the BAS and what does not, and the GST adjustment a write-off can trigger, stated carefully against ATO guidance. The thesis underneath it: keep the finance system your accountant already works from, and feed it accurate stock numbers from the operations layer so these three jobs stop contradicting each other.

The three numbers stock produces, and why people conflate them

Every unit you buy and sell generates three separate accounting facts, and the trouble starts when people assume one of them stands in for the others.

• GST: the goods and services tax you can claim back on a purchase (a GST credit) and the GST you must remit on a taxable sale. This is what your BAS reports.
• COGS: the cost of the stock you actually sold during the period, which reduces your taxable profit. This is an income-tax and profit-and-loss number, not a BAS line.
• Closing stock value: what you still hold at the end of the period, which sits on the balance sheet and rolls into next year's opening value.

The same purchase invoice touches all three: it generates a GST credit now, it adds to your inventory asset now, and it becomes COGS later, only when the item sells. Treat them as one number and your BAS, your P&L and your balance sheet will quietly disagree.

What inventory actually puts on the BAS

The BAS is a GST document first. For most goods businesses on the standard form, only two inventory-related things land on it, and neither of them is COGS.

• G11 (non-capital purchases) and the GST credits at 1B: when you buy taxable trading stock from a GST-registered supplier, the purchase value goes to G11 and the GST you paid is claimable at 1B, in the period the tax invoice falls (subject to your cash or accrual GST basis).
• G1 (total sales) and the GST on sales at 1A: when you sell that stock as a taxable supply, the sale goes to G1 and the GST you collected is reported at 1A.

The thing that surprises people: you claim the GST credit when you buy the stock, not when you sell it. The GST cycle runs on the purchase and sale events, while the cost of that stock only becomes COGS later for income-tax purposes. The two timelines are deliberately different.

Cash vs accrual GST changes the timing, not the logic

Which period a purchase or sale hits your BAS depends on your reporting basis, and stock-heavy businesses feel this acutely.

• On an accrual (non-cash) basis, the GST credit and the GST on a sale fall in the period of the invoice, regardless of when money moves. Stock you have received and been invoiced for generates its credit now, even if you have not paid the supplier.
• On a cash basis, GST follows the payment, so a credit on stock you bought on terms only lands when you actually pay for it.
• Either way, COGS does not move with the BAS. You can have claimed the GST credit on a pallet of stock months before any of it sells and becomes a cost of sale.

Getting the basis right matters because it decides which quarter a large stock purchase swings your refund or liability. Your accountant sets the basis; your job is accurate receipt and sale data, dated correctly.

Why COGS does not belong on the BAS

This is the single most common mix-up, so it is worth stating plainly: cost of goods sold is not reported on the BAS. The BAS reports GST and (where relevant) PAYG, not your profit.

• COGS is calculated as opening stock plus purchases minus closing stock, and it flows to your profit-and-loss statement, reducing the taxable income you report at year end.
• The GST on the purchases that feed COGS was already claimed on the BAS at the time of purchase, item by item, through 1B. You do not get to claim it again as a lump of COGS.
• Inventory you bought but have not sold is not COGS at all yet; it is a balance-sheet asset. Only the cost of what left the building as a sale becomes COGS.

If your closing stock figure is wrong, your COGS and taxable profit are wrong, yet your BAS can still be correct because the GST events were recorded properly. They are independent jobs sharing the same source data, which is exactly why that data has to be trustworthy.

Stock write-offs: the income-tax effect and the GST adjustment

Writing off obsolete, damaged or shrunk stock is where the two systems briefly intersect, and where careful wording matters. There is an income-tax effect and, in specific cases, a separate GST adjustment.

• Income tax: writing down or writing off dead stock reduces your closing stock value, which reduces taxable income. That is a profit-and-loss and income-tax outcome, handled at year end, and it does not by itself appear on the BAS.
• GST on the original purchase: when you bought that stock you almost certainly claimed a GST credit on it. Simply revaluing or writing it off in your accounts does not, on its own, reverse that credit.
• The adjustment to watch: per ATO guidance, an adjustment to your GST credits can arise where the way you actually use stock changes from a creditable purpose, for example if you apply trading stock to private use, give it away, or otherwise use it for a non-creditable purpose. That is reported as an adjustment on your BAS in the relevant period, not as a fresh deduction.

The clean mental model: a paper write-down for income tax is one decision; an increasing GST adjustment because the stock's use changed is a separate decision that may touch your BAS. Keep them apart, document each, and check the disposal route with your accountant before you act. This is general information, not tax advice for your situation.

Shrinkage, scrap and disposal each behave differently

Not every loss is treated the same way, and the operational record you keep decides whether the tax and GST treatment is defensible.

• Shrinkage found at a count reduces the quantity and value you carry, which reduces closing stock and therefore COGS-driven profit. It generally does not, by itself, create a GST event, but you need a record of what disappeared and roughly why.
• Genuinely obsolete stock that you still hold can be valued below cost for income tax if the lower value is reasonable; that is a valuation decision, not a sale, so no GST on its own.
• Physical disposal, donation or sale below market value is where a GST adjustment is most likely to bite. The treatment depends on what you do with the goods, so the disposal method is a decision to make with eyes open, not a clean-up at quarter end.

In every case the variance is only as defensible as your evidence. Aged-stock reports, adjustment reasons and disposal records turn a write-off from a guess into a documented position.

How the data flows into Xero and MYOB

Here is the practical pipeline, because the failure is almost never the ledger; it is the quality and timing of the data arriving in it.

• Receipts: when stock is received and the supplier invoice is entered, Xero or MYOB records the GST credit and increases the inventory asset. If your warehouse receives goods days before the invoice is keyed, your stock-on-hand and your ledger drift apart.
• Sales: when an order ships and invoices, the sale and its GST hit the activity-statement figures, and the cost of those specific units should move from inventory to COGS.
• Adjustments: counts, write-downs and disposals adjust the inventory value, and the income-tax and any GST consequences follow from how each adjustment is classified.

Capture receipts, sales and adjustments accurately and on time at the operations layer, and the ledger does the GST and COGS maths correctly and the BAS reconciles. Reconstruct them from a shoebox at quarter end, and every one of the three numbers above is a guess.

A practical pre-BAS checklist for goods businesses

Before you lodge, run through this so the activity statement reflects reality rather than a hopeful estimate.

• Confirm every stock receipt for the period has a matching supplier invoice entered, so claimed GST credits at 1B line up with stock actually on hand.
• Reconcile shipped-and-invoiced sales against the GST reported at 1A, so nothing taxable slipped through as a non-sale movement.
• Flag any stock applied to private use, given away or disposed of below market value, and check with your accountant whether an increasing GST adjustment is required this period.
• Keep write-downs (income tax) and GST adjustments (BAS) as two separately documented decisions, not one combined entry.
• Make sure stock movements are dated in the period they physically happened, especially around the quarter boundary, so timing matches your GST basis.

Do this from accurate live data, not a frantic reconstruction. The whole point is that the BAS becomes a report you confirm, not a puzzle you solve.

How OpsUI fits

The reason inventory, COGS and write-offs misbehave on the BAS is almost always upstream: the stock data feeding Xero or MYOB is late, incomplete or reconstructed after the fact. OpsUI fixes the upstream by sitting as the operations layer on top of the finance system your accountant already works from, so the GST, COGS and closing-stock numbers all derive from the same accurate movements.

• The Inventory Management module runs true perpetual inventory, so receipts, sales and adjustments update stock value in real time, and the figures behind COGS and your activity statement come from one trusted source rather than a quarter-end guess.
• The Receiving and Inbound module captures goods-in against supplier documents, so GST credits you claim line up with stock you actually hold, and timing matches your GST basis around the quarter boundary.
• Dashboards and Reporting surfaces aged-stock, shrinkage and slow-mover views, so write-down candidates are visible with evidence attached well before you lodge.
• Bidirectional NetSuite sync is live in production today; bidirectional Xero and MYOB sync is wired during rollout through the Finance and Accounting module, so receipts, sales and adjustments land back in the ledger without re-keying.

You keep your ledger and add the operations layer that makes its numbers reliable. Flat modular pricing from A$399/module/mo — full breakdown at /pricing. When you want to see it against your own stock and BAS cycle, book a walkthrough at /book-demo, and if you are heading into the financial year end, pair this with our EOFY guide at /blog/eofy-stocktake-au-2026.

This article is general information about how inventory, COGS and GST flow through the BAS in Australia and is not tax advice. Confirm your GST adjustments and obligations with your registered tax or BAS agent.