Provisional Tax in NZ: How It Works, How to Calculate It, How to Avoid Surprises

If you're self-employed, run a company, or earn untaxed income above $5,000 a year, the IRD wants tax from you in instalments through the year — not just at the end. That's provisional tax. Get it right and it's a non-event. Get it wrong and you'll pay use-of-money interest (UOMI) and possibly penalties on top.

This guide is educational only. The right method for your business depends on how predictable your income is, your industry, and what your accountant recommends.

When provisional tax kicks in

Under the rules in effect 2026, you become a provisional taxpayer the year after you have residual income tax (the tax owed on your annual return) above $5,000. So if your end-of-year tax bill for the 2025–2026 year is $5,001 or more, you're paying provisional tax across the 2026–2027 year.

Residual income tax means the income tax you owe after deducting any PAYE already paid by an employer, withholding tax already deducted, and any tax credits.

The three calculation methods

The IRD lets you pick how to calculate your provisional tax payments. Each has trade-offs.

Standard method (uplift)

Your provisional tax for the current year equals last year's residual income tax plus 5% (or two years ago plus 10% if last year's return is late).

• Easy to calculate
• Most people default to this
• Works badly if your income is dropping — you overpay and wait for refunds
• Works badly if your income is jumping — you underpay and risk UOMI

Based on these inputs, the standard method suits businesses with steady year-on-year growth around 5%.

Estimation method

You estimate your residual income tax for the current year and pay one-third of that estimate at each of the three instalment dates.

• Useful if your income is changing meaningfully year-on-year
• You can revise the estimate during the year (but each revision creates a true-up)
• Underestimate by too much and UOMI applies on the difference
• Requires reasonably reliable forecasting

If nothing changes about your forecasting accuracy, estimation is the right move when you can confidently project full-year residual income tax within ±10%.

Accounting income method (AIM)

AIM is a newer method that ties provisional tax payments to your actual accounting profit, calculated by AIM-capable accounting software (Xero, MYOB, Reckon Solo). You pay provisional tax monthly or two-monthly based on what the software calculates.

• Smooths cash flow — you pay tax on actual results, not estimates
• Avoids UOMI entirely (provided you use a certified AIM provider correctly)
• Available to businesses with turnover under $5 million
• Costs more in software fees and bookkeeping discipline

AIM works well for businesses with seasonal income or unpredictable margins, where the standard or estimation methods would force them to overpay or underpay.

When the instalments are due

If you file GST two-monthly, your provisional tax instalments line up with your GST returns. For most provisional taxpayers on a 31 March balance date, the standard instalment dates are:

Instalment	Standard date	What's due
First (P1)	28 August	One-third of the year's provisional tax
Second (P2)	15 January	One-third more
Third (P3)	7 May	The final third

If your balance date isn't 31 March, the instalment dates shift accordingly. AIM users follow a different schedule — every two months alongside GST.

The use-of-money interest trap

UOMI is the IRD's interest charge when you've underpaid provisional tax. As at 2026, the rate is set above commercial borrowing rates so the IRD effectively penalises you for using their money interest-free during the year.

UOMI applies when:

• You used the estimation method and underestimated
• Your residual income tax exceeds $60,000 and you didn't pay enough at the standard instalments
• You used the standard method but your actual residual income tax materially exceeds last year's plus 5%

For residual income tax under $60,000, the standard method is generally safe — the IRD doesn't charge UOMI on the standard method as long as you paid the standard amounts on time, even if your actual tax owed turns out higher.

Once you cross $60,000, the safe-harbour protection ends. The IRD expects you (or your accountant) to estimate accurately or use AIM.

Avoiding the surprises

The single biggest source of provisional tax pain is the second-year jump. In your first year of self-employment, you probably haven't paid provisional tax — you settle the whole bill in one go at year-end. The next year, you owe both that year's provisional tax AND a top-up for the year just gone. Two years of tax in one window can break a small business's cash flow if it's not anticipated.

If you're entering year two of self-employment, talk to an accountant before P1 hits in August. The right call is usually to set aside roughly one-third of your taxable income each month into a separate account so the instalments don't surprise you.

What to do next

If you're new to provisional tax, the standard method is the safe default — predictable, no UOMI risk under $60,000. Once your business stabilises and you can forecast accurately, AIM or estimation may save you cash flow.

Whichever method you pick, don't try to do this without an accountant in year one. The setup matters and getting it right early is far cheaper than fixing it after the fact.

This is educational content, not personalised tax advice. For your situation, talk to a qualified NZ accountant.