au-compliance-kit

Australian payroll and tax compliance calculations for software (and the AI agents writing it). Zero dependencies.

Live calculators built on it: aftertaxau.com.

npm install au-compliance-kit

• PAYG withholding for 2025–26 and 2026–27: ATO Schedule 1 (NAT 1004), scales 1–6, weekly, fortnightly, monthly and quarterly pays, tax offsets and the Medicare levy adjustment, picked by pay date. Tested against all 432 rows of the ATO's published sample data for each year.

• Seniors and pensioners: the seniors and pensioners tax offset (SAPTO) with the ATO's income tests, Schedule 9 withholding for singles, couples and illness-separated couples, and the higher SAPTO Medicare levy thresholds. Checked against the ATO's worked examples.

• Bonuses and overtime: Schedule 5 Method A withholding on bonuses and commissions (checked against the ATO's worked example), and overtime pay.

• Study loan withholding (Schedule 8, 2026–27): HELP/VSL/SSL/AASL component on each pay, checked against the ATO's worked examples.

• Annual tax estimate for 2025–26, 2026–27 and 2027–28 (estimate): income tax (resident and foreign), low income tax offset, Medicare levy (including the family reduction for couples and sole parents), Medicare levy surcharge (single and family thresholds), compulsory study loan repayments, and the refund or bill against tax withheld. Tested against ATO examples.

• Validators: ABN, ACN and TFN check digits; BSB and account number formats.

Exact integer arithmetic is used throughout, so results match the ATO's rounding rules to the dollar.

For AI agents (MCP server)

AI models trained before 1 July 2026 don't know this year's tax rates, and PAYG withholding uses the ATO's own formulas and rounding rather than the tax brackets. This package includes a Model Context Protocol server so Claude, ChatGPT, Cursor and other agents calculate instead of guessing.

claude mcp add au-tax -- npx -y au-compliance-kit mcp

Other MCP clients:

{ "mcpServers": { "au-tax": { "command": "npx", "args": ["-y", "au-compliance-kit", "mcp"] } } }

Tools: take_home_pay, payg_withholding, bonus_withholding, annual_tax_estimate, super_guarantee, validate_au_identifier and tax_rates. All are read-only and need no network or API key. The same tools are served over HTTP by the calculator site (/mcp for MCP clients, /api/<tool> as plain JSON, /openapi.json for custom GPT actions), and you can mount them in your own app:

import { handleMcpRequest } from "au-compliance-kit/mcp/http"; // Fetch API Request -> Response (Workers, Next.js, Deno, Node 18+)
import { runTool, TOOLS } from "au-compliance-kit/tools";       // or call the tools directly

Related MCP server: itr-mcp

Use

import { calculateWithholding, estimateAnnualTax, validateAbn } from "au-compliance-kit";

calculateWithholding({ earnings: 2026.77, period: "fortnightly", payDate: "2026-11-13" });
// { withholding: 284, scale: 2, taxYear: "2026-27", breakdown: { scaleAmount: 284, taxOffset: 0, medicareAdjustment: 0, studyLoanComponent: 0 }, source: "..." }

estimateAnnualTax({ salary: 90000, deductions: 2500, studyLoan: true });
// { incomeYear: "2026-27", taxableIncome: 87500, cents: { grossTax: 1677000, medicare: 175000, studyLoan: 269580, totalLiability: 2121580, withheld: 2241200, refund: 119620, ... } }

validateAbn("51 824 753 556"); // { valid: true, normalised: "51 824 753 556" }

calculateWithholding throws for pay dates outside 1 July 2025 – 30 June 2027 rather than silently using the wrong year's tables, and for working holiday makers (Schedule 15, not yet supported).

Every function, with examples: docs/api.md.

Not covered yet

Working holiday makers (Schedule 15), back payments (Schedule 5 Method B), study loan withholding for pays before 24 September 2025, termination payments, daily and casual tables, Payday Super calculations, STP Phase 2 reports, and Fair Work award rates.

Develop

npm test             # runs the ATO sample-data tests

Building and deploying the calculator site, ads and releases: docs/maintaining.md.

Disclaimer

General information, not tax advice. Not affiliated with or endorsed by the Australian Taxation Office. ATO material is used under its copyright terms, which allow copying and adaptation without implying endorsement.