How Borrowing Power Is Calculated in NZ
New Zealand banks use an affordability-based approach. The core formula is straightforward: your net monthly income minus estimated living expenses and existing debt repayments gives a monthly surplus. That surplus is the most you can commit to a mortgage repayment. Banks then work backwards from that surplus to find the maximum loan that the repayments would service at the stress-test rate.
The formula for the monthly repayment factor at rate r (monthly) over n months is:
Monthly repayment per dollar = r × (1 + r)^n / ((1 + r)^n − 1)
Maximum loan = Monthly surplus / Monthly repayment per dollar
Worked Example
| Step | Detail | Amount |
|---|
| Gross income | $80,000/yr single applicant | $80,000/yr |
| PAYE tax (2025/26) | $1,470 + $5,950 + $6,600 + $3,300 = $17,320 | -$17,320/yr |
| Net annual income | $80,000 - $17,320 | $62,680/yr |
| Net monthly income | $62,680 / 12 | $5,223/mo |
| Living expenses | Estimated (0 dependants) | -$1,800/mo |
| Existing debt repayments | Car loan etc. | -$500/mo |
| Monthly surplus | | $2,923/mo |
| Stress-test rate | 8.5% pa, 30 years | r = 0.085/12 |
| Repayment factor | r(1+r)^360 / ((1+r)^360 - 1) | $769 per $100k/mo |
| Maximum loan (affordability) | $2,923 / 0.007691 | ~$380,000 |
| DTI cap (6x gross) | $80,000 x 6 | $480,000 |
| Binding estimate | Lower of the two | ~$380,000 |
NZ PAYE Tax Rates (2025/26)
| Taxable Income Band | Rate |
|---|
| $0 to $14,000 | 10.5% |
| $14,001 to $48,000 | 17.5% |
| $48,001 to $70,000 | 30% |
| $70,001 to $180,000 | 33% |
| Over $180,000 | 39% |
The calculator applies these bands to gross income for each applicant. KiwiSaver contributions and the ACC earners levy are not deducted in this estimate; including them would reduce your take-home pay by approximately 3.5 to 5.5% further.
What Is a Stress-Test Rate?
NZ banks do not assess your ability to service a mortgage at the rate you will actually pay. Instead they use a higher test rate, typically 1.5 to 2 percentage points above the lowest current floating rate, to ensure you could still meet repayments if interest rates rise. In 2026 most major NZ banks apply a stress-test rate of approximately 8.0% to 9.0%. This means your assessed maximum loan will be lower than if the bank simply used the current one-year or two-year fixed rate.
The Debt-to-Income (DTI) Cap
Since June 2024, the Reserve Bank of New Zealand has allowed banks to lend no more than 6 times gross annual income to owner-occupiers (and 7 times for investors) under its DTI speed limits. This cap applies to total debt, so if you already have personal loans or credit card balances the amount available for a home loan may be reduced further. The calculator shows the DTI cap alongside the affordability-based estimate and flags whichever is the binding constraint.
Living Expense Benchmarks
Banks use the Household Expenditure Measure (HEM) as a minimum when assessing living costs. If your stated expenses are below HEM, the bank will substitute HEM automatically. Approximate monthly HEM benchmarks used in NZ:
- Single applicant, no dependants: approximately $1,600 to $1,900/month
- Couple, no dependants: approximately $2,200 to $2,600/month
- Each additional dependant: approximately $300 to $500/month extra
The calculator auto-suggests a living expense figure based on your household size; you can override it to reflect your actual spending.
Tips to Increase Your Borrowing Power
- Pay off short-term debt: Every dollar of monthly debt repayment you eliminate translates to roughly $130 more borrowing capacity at 8.5% over 30 years.
- Reduce credit card limits: Banks count your credit card limit (not balance) as potential debt. Halving a $10,000 limit can add $50,000 or more to your assessed loan ceiling.
- Add a co-borrower: A partner's income can substantially increase the combined surplus available for repayments.
- Increase your deposit: A larger deposit reduces the loan needed, which may bring you within the 80% LVR threshold and avoid the low-equity premium.
- Boost income: Overtime, rental income, and Work and Income Family Tax Credit are accepted by most banks if they are regular and documented.
Related Calculators
Sources and method: NZ PAYE tax rates from Inland Revenue (ird.govt.nz, income tax rates 2025/26). Reserve Bank of New Zealand DTI policy (rbnz.govt.nz, debt-to-income ratio restrictions). Bank affordability assessment methodology based on publicly available CCCFA guidance and major NZ bank home loan documents. Stress-test rates are indicative of typical practice in mid-2026; individual banks set their own floors.
This calculator provides a general estimate only and does not constitute financial advice. Actual borrowing limits depend on your full credit profile, the bank's internal policy, and the specific loan product. Consult a registered mortgage adviser or your bank for a precise pre-approval assessment.
