5.1 Australian GST (10%), Pre-GST and Post-GST Prices, and Tiered Tax Tables

Australian Goods and Services Tax (GST) Fundamentals

In Australia, the Goods and Services Tax (GST) is a broad-based consumption tax of $10\%$ levied on most goods, services, and commercial transactions. In a school context, invoices for classroom furniture, laptops, sporting equipment, charter buses, and catering attract $10\%$ GST, whereas certain items—such as fresh unprocessed food in a school canteen or core curriculum tuition fees—are designated GST-free ($0\%$ GST). Because LANTITE numeracy questions frequently present supplier quotes and tax invoices, you must be able to move fluently between GST-exclusive (pre-GST) prices and GST-inclusive (post-GST) prices, and handle mixed invoices where only some line items attract GST.

Adding GST to a Pre-GST (GST-Exclusive) Price

When a supplier quotes a price excluding GST ($P_{\text{excl}}$), the $10\%$ tax is calculated on that base figure and added to it. Because $10\% = 0.10 = \frac{1}{10}$, the GST amount is simply one-tenth of the pre-GST price:

$$\text{GST} = 0.10 \times P_{\text{excl}} = \frac{P_{\text{excl}}}{10}$$

The final GST-inclusive price ($P_{\text{incl}}$) combines $100\%$ of the original price with the $10\%$ tax, reaching $110\%$ of the pre-GST price:

$$P_{\text{incl}} = P_{\text{excl}} + 0.10 P_{\text{excl}} = 1.10 \times P_{\text{excl}}$$

For example, if an audiovisual contractor quotes $1,450 excluding GST to install a classroom projector, the GST component is $1{,}450 \div 10 = 145$ ($145), and the total GST-inclusive invoice is $1{,}450 \times 1.10 = 1{,}595$ ($1,595).

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Extracting GST and Pre-GST Price from a GST-Inclusive Price

When a retail receipt or tax invoice states "Total includes GST", the displayed price $P_{\text{incl}}$ already contains the $10\%$ tax. Writing the forward multiplier as an exact fraction reveals the proportional structure:

$$P_{\text{incl}} = 1.10 \times P_{\text{excl}} = \frac{11}{10} \times P_{\text{excl}}$$

Think of any GST-inclusive price as being composed of $11$ equal parts:

• $10$ parts ($\frac{10}{11}$ of the total) make up the original pre-GST price ($P_{\text{excl}}$).
• $1$ part ($\frac{1}{11}$ of the total) makes up the embedded GST.

Consequently, to extract the GST or recover the pre-GST price from a GST-inclusive figure, you divide by $11$ or $1.10$:

$$\text{Embedded GST} = \frac{P_{\text{incl}}}{11}$$ $$P_{\text{excl}} = \frac{P_{\text{incl}}}{1.10} = \frac{10}{11} \times P_{\text{incl}} = P_{\text{incl}} - \text{Embedded GST}$$

Avoiding the #1 LANTITE GST Trap

The most common financial numeracy error on the test is calculating $10\%$ of a GST-inclusive price and subtracting it. Consider a tax invoice for $330 including GST:

• Correct method: Divide by $11$ to find the embedded GST: $330 \div 11 = 30$ ($30). Subtract that $30 (or multiply by $10$) to obtain the true pre-GST price of $300. You can verify this immediately because adding $10\%$ GST to $300 gives $300 + 30 = 330$ ($330).
• Incorrect trap method: Taking $10\%$ of $330 gives $33 in GST and a false pre-GST price of $330 - 33 = 297$ ($297). If the pre-GST price were truly $297, adding $10\%$ GST would produce $297 + 29.70 = 326.70$ ($326.70), which does not match the $330 invoice.

Worked Example 1: Mixed Invoice with Taxable and GST-Free Items

A school science coordinator receives an invoice for $1,900 in total. The invoice consists of laboratory glassware priced at $1,760 including GST, and $140 of GST-free distilled water and safety charts. How much GST is included on the invoice, and what is the total pre-GST value of the order?

1. Isolate the GST-inclusive items: Only the $1,760 glassware attracts GST. The $140 items have $0 GST.
2. Find the embedded GST: Divide the GST-inclusive amount by $11$:

$$\text{GST} = \frac{1{,}760}{11} = 160$$

So the embedded GST is $160. 3. Calculate the total pre-GST value: Subtract the $160 GST from the $1,900 invoice total (or add the $1,600 pre-GST glassware to the $140 GST-free items) to obtain $1,740.

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Reading Australian Progressive Income Tax Tables

Australian resident personal income tax is calculated on taxable income, which is an employee's gross income from all sources (salary, bank interest, and allowances) minus allowable tax deductions (such as teacher registration fees, union dues, charitable donations, and work-related reference books):

$$\text{Taxable Income} = \text{Gross Income} - \text{Allowable Deductions}$$

Australia uses a progressive (marginal) tax system. When your taxable income moves into a higher bracket, the higher percentage rate applies only to the dollars earned above that bracket's threshold, not to your entire income. The fixed dollar figure listed at the start of each bracket row simply represents the cumulative tax already owed on all lower brackets combined.

The table below shows the Australian Taxation Office (ATO) resident rates for 2026–27 (the Medicare levy is not included). Rates change between income years—the 2024–25 and 2025–26 tables used $16\text{c}$ in the second row and a $4,288 base in the third—so always use the table printed in the question.

Notice where the base figure $4,020 in the third row comes from: the entire second bracket spans $45{,}000 - 18{,}200 = 26{,}800$ ($26,800), and taxing that slice at $15\%$ ($0.15$) yields $26{,}800 \times 0.15 = 4{,}020$ ($4,020).

Adding the Medicare Levy ($2\%$) and Checking Refunds

In addition to base income tax, most Australian residents pay the Medicare levy, which is a flat $2\%$ ($0.02$) of their entire taxable income (not merely the excess over a threshold). Unless a question states that an exemption or surcharge applies, total tax liability is the sum of base income tax and the $2\%$ Medicare levy. If an employer has already withheld Pay As You Go (PAYG) tax instalments across the year, comparing total PAYG tax withheld against the final tax liability determines whether the teacher receives a tax refund (when PAYG withheld exceeds total tax liability) or owes a tax bill (when PAYG withheld is less than total tax liability).

Worked Example 2: Graduate Teacher Tax and Medicare Levy

A graduate teacher earns a gross annual salary of $84,200 and claims $1,700 in allowable work-related deductions. Using the tax table above, calculate their total income tax and $2\%$ Medicare levy.

1. Find taxable income: $84{,}200 - 1{,}700 = 82{,}500$ ($82,500).
2. Identify the tax bracket: $82,500 falls in the $45,001 – $135,000 bracket ("$4,020 plus $30\text{c}$ for each $1 over $45,000").
3. Calculate base income tax:

$$\text{Income Tax} = 4{,}020 + 0.30 \times (82{,}500 - 45{,}000) = 4{,}020 + 0.30 \times 37{,}500 = 4{,}020 + 11{,}250 = 15{,}270$$

The base income tax is $15,270. 4. Calculate the $2\%$ Medicare levy on total taxable income:

$$\text{Medicare Levy} = 0.02 \times 82{,}500 = 1{,}650$$

1. Sum for total tax payable: $15{,}270 + 1{,}650 = 16{,}920$, or $16,920.

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Summary of GST and Tax Operations