Second Job / Side Hustle Tax Bill at IR3 Time 2025-26 & 2026-27 — Why You Owe Money (IRD)

Two jobs. Side-gig Uber on Friday nights. Selling on Etsy alongside the day job. All of these create the single most common cause of unexpected NZ tax bills at year-end: secondary-income under-withholding.

This guide explains how NZ’s M + S/SH/ST/SA tax code system under-withholds, why the problem only surfaces at IR3 or automatic-assessment time, and how to avoid the wash-up shock in 2026-27. Covers 2025-26 and 2026-27.

Want a fast sanity check? Our tax code checker recommends the correct code based on your combined income. The take-home pay calculator lets you confirm the number against your payslip.

The Mechanics — Why Secondary Codes Under-Withhold

NZ has one set of marginal brackets and two different withholding systems sitting on top of them.

The brackets (2025-26 & 2026-27 same structure):

Income range	Marginal rate
$0 – $15,600	10.5%
$15,601 – $53,500	17.5%
$53,501 – $78,100	30%
$78,101 – $180,000	33%
Over $180,000	39%

The primary code (M or ME): your main job is paid as if it were your only income. The employer applies the progressive brackets — you get the tax-free-ish slice through the bottom bracket first, then step up.

The secondary codes (SB / S / SH / ST / SA): flat rates applied to every dollar the secondary employer pays you. IRD tells you which rate to pick based on total combined income from all sources:

Code	Rate	Use when combined income is…
SB	10.5%	$15,600 or less
S	17.5%	$15,601 – $53,500
SH	30%	$53,501 – $78,100
ST	33%	$78,101 – $180,000
SA	39%	Over $180,000

If you pick the right secondary code and nothing changes during the year, withholding is reasonably close to correct. The problems start when:

1. You pick the wrong secondary code (very common — the IR330 form relies on you forecasting combined income correctly)
2. You pick the right one but circumstances change — bonus, pay rise, new side gig — mid-year
3. You’re close to a bracket boundary and the secondary code lags by one tier
4. Your secondary employer uses S by default when you hand them an IR330 without specifying one

The Bracket-Boundary Trap

Here’s the part that catches people out. Say your primary job earns $60,000 PAYE and you pick up a $15,000 secondary role. Combined income is $75,000.

• Primary $60k → tax on $60,000 using progressive brackets = $10,420 income tax (ignoring IETC for simplicity).
• Secondary $15k at code SH (30%) → withholds $4,500.
• Total PAYE withheld: $14,920.

Your actual tax owing on $75,000 combined: $15,745 (progressive brackets applied to full amount).

Shortfall: $825 — a small but annoying tax bill at year-end.

Now imagine the secondary employer used S (17.5%) instead (the default when nothing else is specified on IR330):

• Secondary withholds: $15,000 × 17.5% = $2,625
• Total PAYE withheld: $13,045
• Tax actually owing: $15,745
• Shortfall: $2,700

Nothing changed except the code selection on one form, and the bill jumped from $825 to $2,700.

At $78,100 – $180,000 combined (ST code, 33% rate), the gap widens further — every dollar of secondary income pays 33%, but the real marginal rate on that dollar is 33% of some portion and 30% of the portion below the bracket boundary. Flat rates can’t thread that needle.

Three Worked Examples

Example 1 — $70k Combined, Small Bill ($400–$600)

Aroha. Primary job $55,000 at M code. Evening retail second job $15,000, secondary code SH (30%) (her myIR dashboard correctly flagged SH because combined is $70k, which sits in the $53,501–$78,100 band).

• Primary PAYE: ~$8,860 withheld (standard M code on $55k)
• Secondary PAYE: $15,000 × 30% = $4,500 withheld
• Total withheld: $13,360
• Actual tax on $70,000 combined: $13,945
• Shortfall at IR3 time: ~$585

Small bill, manageable. She updates her estimate in myIR for next year and considers requesting a Special Tax Code if the arrangement continues.

Example 2 — $95k Combined, $1,400 Bill from Wrong Code

Rohan. Primary job $80,000 at M, side consulting $15,000 which he treated as a second PAYE job. He filled in the IR330 quickly and ticked S without looking up the combined figure.

• Primary PAYE: ~$15,550 withheld
• Secondary PAYE: $15,000 × 17.5% = $2,625 withheld
• Total withheld: $18,175
• Actual tax on $95,000 combined: $19,545
• Shortfall at IR3 time: $1,370

He should have used ST (33%) because combined income is in the $78,101–$180,000 band. ST would have withheld $4,950 from secondary, total $20,500, and given him a small refund of ~$955 instead of a bill.

Example 3 — $140k Combined, $3,200 Bill at ST/Wrong Code

Mei. Primary job $110,000 at M, weekend private tutoring $30,000 at secondary code SH (30%) (she picked SH because the form listed it as “30% bracket”, without realising her combined income had moved her into the 33% bracket).

• Primary PAYE: ~$25,420 withheld
• Secondary PAYE: $30,000 × 30% = $9,000 withheld
• Total withheld: $34,420
• Actual tax on $140,000 combined: $37,595
• Shortfall at IR3 time: $3,175

Correct code was ST (33%). With ST, secondary withholding would have been $9,900, total $35,320 — still a $2,275 shortfall because the flat ST rate under-withholds below the $180k bracket. For Mei, a Special Tax Code requested via myIR would have set the exact rate at around 36% and wiped out almost all of the year-end bill.

How to Prevent the Bill in 2026-27

1 — Use the right code from day one. When you fill in an IR330 for your secondary employer, include every source of income. Use our tax code checker with your total expected annual income, not just the secondary job’s number.

2 — Apply for a Special Tax Code if you’re in the 30%+ bracket (combined income over $53,500). IRD sets a precise withholding rate that matches your marginal rate almost exactly. Apply in myIR → More → Tax codes → Request a special tax code. Takes 5–10 business days to issue. STC is re-issued annually — check the new certificate each April.

3 — Update the code mid-year when circumstances change. New contract, bonus, pay rise, redundancy, return to work, extra side gig — re-run your combined income estimate and hand your secondary employer a new IR330. Takes one pay cycle to bed in.

4 — Set aside the likely shortfall in a separate account. If you know ST under-withholds your secondary income by ~3-4%, put 3% of every secondary pay into a “tax bill” account. The interest earns you a little, and when the bill arrives you’re ready.

5 — Treat schedular / contract income differently. If your secondary income is invoices not payslips, the withholding rule is different — you’ll either elect a rate on IR330C or use the standard per-industry WT rate. Payroll doesn’t handle that. Schedular work goes through the IR3 schedular payments section, and reconciliation happens there. Read our schedular payments guide for that track.

What Happens at IR3 / Auto-Assessment Time

If all your income is PAYE (primary + secondary), you usually get an automatic assessment from late May and the shortfall shows up as a bill on the notice. You can pay via myIR straight away or set up an instalment arrangement.

If you had any non-PAYE income (self-employment, rental, overseas, bright-line), you must file an IR3 and the secondary-income shortfall shows up as part of the IR3 wash-up. Terminal tax due 7 February 2027 (self-filers) or 7 April 2027 (tax agent clients).

IRD charges Use of Money Interest (UOMI) on unpaid tax at 10.04% p.a. (Q2 2026) from the original due date, but not on amounts settled inside the 30-day period after the notice of assessment. If you pay promptly on receipt there’s no interest cost.

Sources

• IRD — Tax codes and tax rates for individuals
• IRD — Secondary tax codes
• IRD — Special tax code (tailored tax code)
• IRD — Use of money interest (UOMI)

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Fix your secondary code before the next IR3 surprise

If you have two jobs or a regular side hustle, five minutes on the tax code checker can save you a four-figure wash-up bill. For anyone in the 30%+ brackets with multi-source income, request a Special Tax Code in myIR — it’s the single cleanest way to eliminate year-end surprises in 2026-27.