Provisional tax is not a separate tax. It is income tax paid in instalments during the year rather than in one lump afterwards. That distinction matters, because most of the pain businesses feel around provisional tax comes not from the amount but from the method they were signed up to without ever choosing it.
When you become a provisional taxpayer
You are generally required to pay provisional tax for a year if your residual income tax for the previous year exceeded the threshold set by Inland Revenue. Residual income tax is broadly your tax liability after credits such as PAYE already deducted.
The practical trigger for most small businesses is a first profitable year. You file a return, discover tax is owed, and simultaneously learn that instalments toward next year’s tax have already started falling due. The double hit in that first year is the single most common cashflow shock in New Zealand small business, and it is entirely predictable if someone warns you.
The three calculation options
Standard method. The default. You pay last year’s residual income tax plus an uplift percentage, spread across instalments. It is simple and it protects you from use-of-money interest provided you pay on time.
Its weakness is that it looks backwards. A business that grew sharply pays instalments based on a smaller prior year, then faces a large terminal payment. A business that shrank pays instalments based on a bigger prior year, effectively lending Inland Revenue money it will get back later.
Estimation method. You estimate this year’s liability and pay accordingly. Useful when you know income has changed materially — a contract ended, a major asset was sold, a new revenue line started.
The risk is real. Estimate too low and use-of-money interest applies to the shortfall, calculated from each instalment date. Estimation suits businesses with genuine visibility over their year, not businesses hoping for the best.
Accounting Income Method (AIM). Available to smaller businesses using approved accounting software. AIM calculates provisional tax from actual year-to-date results at each instalment, so you pay tax on profit you have genuinely made.
For seasonal and volatile businesses this is a considerable improvement. If you make nothing in a period, you pay nothing for that period. The trade-off is discipline: your accounts must be genuinely current, coded correctly, and reconciled at every instalment date. AIM rewards businesses that keep tidy books and punishes those that catch up quarterly.
Use-of-money interest, and why it bites
Where you underpay, Inland Revenue charges interest from the date the money should have been paid. Where you overpay, interest runs in your favour, at a lower rate. The asymmetry is deliberate.
Interest is what turns a manageable tax bill into an unpleasant one, and it is almost always avoidable. Paying under the standard method on time gives statutory protection for most taxpayers; estimating badly removes it.
Choosing between them
- Stable, predictable income — standard method. There is no advantage in complexity.
- Income clearly down on last year — estimation, so you are not funding instalments against income you will not earn.
- Income clearly up — stay on standard, but put money aside for the terminal payment. The uplift will not cover it.
- Seasonal or lumpy income, tidy books — AIM is worth a serious look.
- Books perpetually three months behind — not AIM. It will make things worse.
The habit that solves most of it
Whatever method you use, the businesses that never have a provisional tax problem all do the same thing: they move a fixed percentage of every payment received into a separate account, and they leave it there. Inland Revenue’s own guidance recommends the approach, and it works because it removes the decision entirely.
Provisional tax dates are published well in advance at ird.govt.nz, and accounting software will map them for you. There is no version of this that is improved by finding out late.
General information only, not tax advice. Your situation may differ — check with Inland Revenue or your accountant.
