The Goods and Services Tax (GST) is a flat 10% tax applied to most goods and services sold in Australia. Unlike some countries with varying regional sales tax rates, GST is a single national rate — there's no difference between states or territories.
To add GST to a GST-exclusive price, simply multiply by 10% (or 0.10) and add the result to the original amount. For example, a $100 GST-exclusive price becomes $110 once GST is added.
To find the GST component already included in a GST-inclusive price, divide the total by 11 (not 10) — since the GST-exclusive amount plus 10% GST equals 11 parts of the whole. For example, a $110 GST-inclusive price contains exactly $10 of GST, and the GST-exclusive amount is $100.
Businesses with a GST turnover of $75,000 or more per year (or $150,000 for non-profit organisations) must register for GST with the ATO. Businesses below this threshold can register voluntarily, which allows them to claim GST credits on their business purchases but also requires them to charge and remit GST on their sales.
Not everything attracts GST. Most basic food items, many healthcare and medical services, and most education services are GST-free. Financial services and residential rent are typically "input-taxed" instead, which is treated somewhat differently for GST credit purposes than a straightforward GST-free classification.
Adding GST: $100 × 10% = $10 GST, for a total of $110. Removing GST from a $110 GST-inclusive price: $110 ÷ 11 = $10 GST, meaning the GST-exclusive amount is $100.