Introduction

For Australian businesses registered for GST, issuing a mixed invoice—one that includes both taxable supplies (10% GST) and GST-free supplies—is a common scenario. Whether you run a café selling both food and takeaway coffee, a bookshop selling educational materials and stationery, or an importer handling goods that cross GST-free thresholds, understanding how to correctly calculate and report GST is essential. Incorrect treatment can lead to ATO penalties, cash flow issues, or missed input tax credits. This comprehensive guide explains the principles, provides step-by-step methods, and offers practical tools to ensure your mixed invoices are GST-compliant.

1. Understanding Mixed Invoices

What Is a Mixed Invoice?

A mixed invoice is a tax invoice that includes both taxable supplies (subject to 10% GST) and GST-free supplies (not subject to GST). The invoice must clearly separate these items so that GST is calculated only on the taxable portion. The ATO requires that each supply be individually identified and that the total GST amount be shown separately.

Why It Matters

Correctly handling mixed invoices affects your Business Activity Statement (BAS) reporting. You must report GST on taxable sales (G1) and claim input tax credits on purchases (G2). Errors can result in underpayment or overpayment of GST, leading to audits or missed refunds.

2. Identifying GST-Free Supplies

Common GST-Free Categories

Under the A New Tax System (Goods and Services Tax) Act 1999, certain goods and services are GST-free. Key categories include:

Basic food – unprocessed food items (e.g., fruit, vegetables, meat, bread, milk, eggs).
Health and medical – most medical services, prescribed medicines, and health-related aids.
Education – courses accredited under the Australian Qualifications Framework (AQF).
Childcare – approved childcare services.
Exports – goods exported from Australia within 60 days of sale.
Religious and charitable supplies – certain supplies by non-profit bodies.
Water, sewerage, and drainage – supplied through mains.
Farm supplies – certain agricultural products and inputs.

How to Determine GST Status

Use the ATO’s GST classification tool or refer to the specific legislation. For example, a loaf of bread is GST-free, but a pre-packaged sandwich with added ingredients may be taxable if it is considered “prepared food”. Similarly, a book may be GST-free if it is a school textbook, but a general fiction novel is taxable. Always check the latest ATO rulings.

Expert Tip: When in doubt, treat the supply as taxable. You can later adjust if you receive a ruling or advice from a registered tax agent. Overpaying GST is better than underpaying and facing penalties.

3. Step-by-Step Calculation Method

Method 1: Separate Line Items

The simplest and most accurate method is to list each item on the invoice with its GST status and price (including or excluding GST). Then calculate GST only on the taxable items.

List all items with their GST-free or taxable status.
For taxable items, record the price excluding GST (or include GST and divide by 11 to find the GST component).
Sum the total of taxable supplies (exclusive of GST).
Multiply that total by 10% to get the GST amount.
Add the GST amount to the total of all supplies (taxable + GST-free) to get the total invoice amount.

Example Calculation

A café sells a coffee ($5.50 including GST) and a banana ($1.00 GST-free). The coffee is taxable, the banana is GST-free.

Item	Price (incl. GST)	GST Status	Price excl. GST	GST
Coffee	$5.50	Taxable	$5.00	$0.50
Banana	$1.00	GST-free	$1.00	$0.00
Total	$6.50		$6.00	$0.50

GST = $5.00 × 10% = $0.50. Total invoice = $5.50 + $1.00 = $6.50.

Method 2: Apportionment (Not Recommended)

Some businesses attempt to apportion a single total amount between taxable and GST-free. This is only acceptable if the invoice is for a single supply that has both taxable and GST-free components (e.g., a mixed basket of goods sold as one item). The ATO generally requires separate identification. Avoid apportionment unless you have a clear basis and documentation.

4. Adjustments for Discounts, Refunds, and Credits

Discounts on Mixed Invoices

If you offer a discount on the total invoice, you must allocate the discount proportionally between taxable and GST-free supplies. The ATO’s preferred method is to reduce the price of each supply in proportion to its original value. For example, a 10% discount on a $100 invoice ($60 taxable, $40 GST-free) reduces the taxable portion to $54 and the GST-free to $36. GST is then recalculated on $54.

Refunds and Credit Notes

When a customer returns a mixed invoice item, issue a credit note that adjusts the GST accordingly. If the returned item was taxable, reduce the GST amount. If GST-free, no GST adjustment is needed. Ensure the credit note references the original invoice and clearly states the GST adjustment.

5. Record-Keeping Requirements

What to Keep

The ATO requires you to retain records for at least five years. For mixed invoices, keep:

Copies of all tax invoices issued (including those with mixed supplies).
Supporting documents showing the GST status of each item (e.g., supplier statements, product classifications).
Calculations showing how GST was determined.
Credit notes and adjustments.

Digital Records

Use accounting software that supports GST codes (e.g., Xero, MYOB, QuickBooks). Assign a GST code to each line item: “GST” for taxable, “GST Free” for exempt. This automates calculations and reduces errors.

6. Special Considerations for Importers

GST on Imported Goods

Importers must pay GST on most imported goods at the time of customs clearance (unless the goods are GST-free or the value is below $1,000 for low-value imports). When reselling, the importer issues a mixed invoice if some items are GST-free (e.g., imported basic food). The GST paid at customs can be claimed as an input tax credit on the BAS, provided the importer is GST-registered.

Example for Importers

An importer brings in a shipment of organic quinoa (GST-free basic food) and packaged snacks (taxable). The customs value is $10,000 for quinoa and $5,000 for snacks. GST paid at customs: $5,000 × 10% = $500 (only on taxable snacks). When selling, the invoice must separate the two categories. The importer claims the $500 input tax credit on the BAS.

7. Using the GST Calculator on gstcalculatorau.com

Step-by-Step Guide

Our free GST calculator suite simplifies mixed invoice calculations. Follow these steps:

Go to gstcalculatorau.com and select the “Mixed Invoice Calculator”.
Enter the total amount of taxable supplies (excluding GST) in the first field.
Enter the total amount of GST-free supplies in the second field.
Click “Calculate”. The tool instantly shows the GST amount, total invoice including GST, and a breakdown.
Use the “Export” feature to generate a summary for your records.

Sample Calculation Table

Input	Value
Taxable supplies (excl. GST)	$1,200.00
GST-free supplies	$800.00
GST (10% of taxable)	$120.00
Total invoice (incl. GST)	$2,120.00

The calculator also handles discounts and multiple line items. For complex invoices, use the “Line Item” mode.

8. Common GST Mistakes to Avoid

Applying GST to GST-free items – e.g., charging GST on basic food. This overcharges customers and creates reporting errors.
Not separating items on the invoice – lumping all items together without indicating GST status. The ATO may reject the invoice as a valid tax invoice.
Using the wrong GST rate – some items are 0% (GST-free) but others are input-taxed (e.g., residential rent). Input-taxed supplies are different and cannot be claimed for input tax credits.
Incorrect discount allocation – applying the discount only to taxable items or arbitrarily. Use proportional allocation.
Failing to adjust for refunds – not issuing a credit note with correct GST adjustment.
Ignoring import GST – not claiming input tax credits for GST paid at customs on taxable imports.
Not keeping adequate records – missing documentation can lead to ATO adjustments during an audit.

Conclusion

Calculating GST on mixed invoices requires careful identification of taxable and GST-free supplies, accurate arithmetic, and proper record-keeping. By following the methods outlined in this guide—separate line items, proportional discount allocation, and using reliable tools—you can ensure compliance and avoid costly mistakes. For quick and error-free calculations, use the GST Calculator Suite at gstcalculatorau.com. Our searchable database also helps you determine the GST status of thousands of goods and services. Remember, this information is general in nature; always consult a registered tax agent for your specific circumstances.