Sources & Methodology
This page explains the sources and calculation approach used by ANZ GST Calculator.
Primary Sources
For New Zealand GST rules and administrative information, we use Inland Revenue (IRD) as the primary government source.
For Australian GST rules and administrative information, we use the Australian Taxation Office (ATO) as the primary government source.
Calculation Method
The basic calculators perform arithmetic using the GST rate stated on the tool. For standard New Zealand GST, the rate used is 15%. For standard Australian GST, the rate used is 10%.
New Zealand
- Add GST: amount × 0.15
- Remove GST from an inclusive amount: inclusive amount ÷ 1.15
- GST component of an inclusive amount: inclusive amount × 3 ÷ 23
Australia
- Add GST: amount × 0.10
- Remove GST from an inclusive amount: inclusive amount ÷ 1.10
- GST component of an inclusive amount: inclusive amount × 10 ÷ 110
Scope and Limitations
These formulas cover standard GST arithmetic only. They do not decide whether a transaction is taxable, GST-free, input-taxed, exempt, deductible, refundable, or subject to a special rule. Those questions require the facts of the transaction and the applicable official guidance.
Updates
When a tax authority changes a rate, threshold, filing rule, or other requirement relevant to a tool, we review the affected page and update the calculation or explanation where necessary.
Verification and review: every figure used in our calculators and worked examples, including GST rates, registration thresholds, and filing rules, is checked against the official guidance published on ird.govt.nz for New Zealand and ato.gov.au for Australia before a page goes live. We do not take figures from news stories or other calculator sites. Pages are re-checked whenever Inland Revenue or the ATO announces a relevant change, and we run a full site-wide review each year so outdated numbers do not stay online.