Methodology & Sources

Source authorities

All tax rules, thresholds, and rates used across NZTaxTools calculators come directly from Inland Revenue (IRD) and other New Zealand government publications (MBIE for KiwiSaver, MSD for benefit-related figures).

Update cadence

We review calculators at the start of each tax year (1 April), on Budget day when changes are announced, and quarterly for administrative rates that move between years (use-of-money interest, FBT prescribed rate). Each calculator page shows a "Last updated" date generated from the page's revision history, falling back to the build date where no history is available. In July 2026 we completed a full-site accuracy review — every calculator, reference page, and article re-checked against primary sources. The full policy, including how corrections are handled, is in our editorial policy.

Where the math lives

Calculation logic lives in tested TypeScript files under src/utils/, separated from UI, so every number can be traced back to a tested function. The site currently runs 80+ calculators across roughly 380 pages, backed by an automated suite of over 1,500 tests — including anchor tests pinned to published IRD figures that fail loudly if a rate is changed accidentally. Rates and thresholds are defined once in central data files, so one verified update flows through every page that uses the figure, and every build checks internal links sitewide before it deploys.

Handling ambiguous rules

Where IRD guidance is ambiguous or depends on facts we can't observe (residency mid-year, partial-year tax credits, contract vs. employee classifications), calculators use the most common interpretation and flag the assumption inline.

Not financial advice

NZTaxTools provides general information only. It is not tax, legal, or financial advice. For a binding answer, speak to a qualified accountant or tax adviser.