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) and again when IRD publishes changes between years. Each calculator page shows its own "Last updated" date from the file's last git commit.

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.

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.