Nothing derails a tradie's cash flow quite like a surprise IRD bill. Provisional tax is the main culprit — a system designed to make self-employed people pre-pay next year's tax before they've even earned the income. Confusing? Yes. Avoidable? Only if you understand it.
Here's how it works and what you need to do.
What Is Provisional Tax?
When you're an employee, your employer deducts PAYE from every pay — so your tax is paid automatically throughout the year. When you're self-employed, nobody does that for you. You invoice, get paid, and at the end of the year IRD says "you owe $X in income tax."
Provisional tax is IRD's way of collecting that tax in instalments during the year before you file. Think of it as paying tax on this year's income, using last year's income as the estimate.
You become a provisional taxpayer when your residual income tax (RIT) is $5,000 or more in a tax year. RIT is your total income tax minus any PAYE already paid and any tax credits. For most sole trader tradies, once you're earning around $40,000–$50,000 net, you'll be paying provisional tax.
Three Ways to Calculate Provisional Tax
IRD gives you three methods. Your accountant will usually recommend one, but it's worth understanding each.
Standard Method (Most Common)
You pay 105% of last year's RIT, split across instalment dates. So if you owed $8,000 in tax last year, you'll pay $8,400 in provisional tax this year (3 instalments of $2,800 each).
This is simple but can hurt when your income grows significantly year-on-year — you're always playing catch-up.
Estimation Method
You estimate what this year's tax will be and pay that amount instead. If you think you'll earn more than last year, this lets you pay more and avoid underpayment interest. If you think you'll earn less (quiet year, illness, slow season), you can pay less.
The risk: if you under-estimate, IRD charges use-of-money interest (currently 10.39% p.a.) on the shortfall. Get it right and you save cash in the short term.
Ratio Method
Available to GST-registered businesses. Your provisional tax is calculated as a percentage of your GST taxable supplies each time you file a GST return. You pay provisional tax at the same time as GST — so payments are more frequent but smaller and directly tied to actual turnover.
Many tradies with lumpy income prefer this because you're never far behind your actual earnings.
Provisional Tax Payment Dates
For most NZ individuals (standard balance date of 31 March), provisional tax falls due on:
| Instalment | Due Date |
|---|---|
| 1st | 28 August |
| 2nd | 15 January |
| 3rd | 7 May |
These dates don't shift, so put them in your calendar now. Missing a payment doesn't trigger a penalty immediately (IRD changed this in 2017), but underpayment interest accrues from the due date.
If you have a tax agent (accountant), your terminal tax date (final payment) is usually extended to 7 April following the year end.
The Cash Flow Reality
The biggest trap for growing tradies: you have a good year, you spend the profits, and then in August the first provisional tax bill arrives.
The money you earned is gone. Now you have to find it.
The fix is simple but requires discipline: treat tax as a cost of doing business from day one.
A practical approach: each time you invoice, set aside 25–30% in a separate account labelled "Tax & GST." This covers: - Income tax (provisional and terminal) - GST you've collected on behalf of IRD - ACC levies
You never "have" that money — it lives in the tax account until the due dates. Some tradies use a separate bank account; others use a sub-account within their main business account.
What Happens If You Miss a Payment
Missing provisional tax doesn't trigger an immediate penalty, but use-of-money interest (UOMI) applies to any shortfall. At 10.39% per annum (as of 2026), an $8,000 underpayment over 6 months costs around $415 in interest — not catastrophic but avoidable.
IRD can also charge late payment penalties (5% immediate + 1% monthly) if you have overdue tax debt on your account. Keeping communication open with IRD and having a tax agent usually helps avoid the worst outcomes.
The AIM Method (For Software Users)
If you use approved accounting software (Xero, MYOB, etc.), you may qualify for the Accounting Income Method (AIM). AIM calculates your provisional tax each period based on your actual profit in that period — no estimation, no catch-up.
You pay little when business is slow and more when it's good. For tradies with seasonal income or irregular earnings, AIM can significantly improve cash flow management. Ask your accountant if it makes sense for you.
Key Takeaways
- Provisional tax kicks in when your end-of-year tax bill is $5,000 or more
- Standard method: you pay 105% of last year's tax in three instalments
- Due dates are fixed: 28 August, 15 January, 7 May (standard balance date)
- Set aside 25–30% of income as it arrives — treat it as a cost, not a savings decision
- Missing payments doesn't trigger penalties immediately but interest accrues at 10.39% p.a.
- Talk to an accountant when you first become a provisional taxpayer — the methods matter
Use our free Job Cost Calculator to check your actual margins, and read our NZ Tradie Tax Guide for a full breakdown of deductions available to you.
NZ Tradie Tools provides free calculators, templates, and guides for New Zealand tradies. Always consult a registered tax agent or accountant for advice specific to your situation.