How much can I actually borrow? Test rates explained

What is a mortgage test rate?

A mortgage test rate—also called an assessment or serviceability rate—is an interest rate used inside a lender's affordability calculation. It is not a product rate, forecast or promise about where interest rates are heading. It is a way to calculate a more demanding repayment before the lender decides whether the proposed debt fits your household cash flow.

The test normally assumes principal-and-interest repayments over an accepted loan term. The lender then combines that repayment with its view of your usable income, household expenses and existing commitments. If the calculation does not leave the required surplus, the serviceable loan amount may be reduced even when your deposit is large enough.

New Zealand banks set their own test rates and wider servicing rules. Some may use a fixed floor, a margin above the proposed rate, or whichever of those assumptions is higher. The exact method is part of lender policy and can change without matching an OCR or advertised-rate movement.

Why does the bank test you above the rate you may pay?

A mortgage can last 20 or 30 years, but a special fixed rate might last only six months, one year or two years. When that period ends, the loan will be refixed or move to another available rate. The lender therefore needs to consider more than whether today's initial repayment fits.

• Interest rates can rise. The buffer tests whether the budget has room for a higher repayment after the initial term.
• Household costs change. Rates, insurance, food, transport and other essentials may increase while the mortgage is still running.
• Income is not always perfectly stable. Overtime, bonuses, commission, rent and self-employed earnings may be treated cautiously.
• Lenders have responsible-lending duties. They need to assess whether the credit is suitable and likely to be affordable, not simply whether the first payment can be made.
• The bank is managing long-term risk. Serviceability buffers reduce the chance that a normal rate cycle turns an already-maxed budget into hardship.

The Reserve Bank has noted that servicing-test buffers have helped many borrowers absorb higher mortgage costs. The buffer is not a guarantee that repayments will always be comfortable, but it is a deliberate layer of resilience in the approval process.

What does the test rate do to your borrowing number?

The mathematical effect is straightforward: at the same loan amount and term, a higher interest rate creates a higher calculated repayment. If the lender has decided that only a certain amount of monthly income is available for the new mortgage, the higher repayment assumption supports a smaller principal.

This is why a small movement in the assessment rate can make a noticeable difference to an online borrowing estimate. It is also why the calculator's number should be treated as a range for planning—not as money you can bid with unconditionally.

The test rate is only one part of the affordability assessment

It is tempting to think that borrowing power is simply income minus a test-rate repayment. In practice, the lender makes several policy choices before it reaches the result.

1. Usable income. Base salary may be straightforward, while overtime, bonuses, commission, rent, boarder income and self-employed profit may need a track record or be reduced for assessment.
2. Tax and deductions. The lender considers the money available after tax and deductions such as student-loan repayments.
3. Living costs. Declared expenses may be compared with transaction history and lender benchmarks. Normal costs cannot simply be set to zero.
4. Existing commitments. Vehicle finance, personal loans, maintenance, buy-now-pay-later accounts, overdrafts and other obligations use some of the available surplus.
5. Credit limits. A credit card or overdraft can affect the calculation even when little or nothing is owing, because the facility can be used after approval.
6. Loan term and repayment type. A shorter accepted term creates a higher monthly repayment. Interest-only treatment, where available, is also lender- and purpose-specific.
7. Required surplus. Passing the raw repayment calculation may not be enough; the lender can require money to remain after assessed commitments.

For the wider picture, read our guide to how lenders calculate home-loan borrowing capacity or start with the borrowing power calculator.

Why can two lenders give you different borrowing estimates?

Lenders do not all use the same test rate, income policy, expense model or credit settings. One bank may accept more of a regular allowance or rental income; another may use a different repayment commitment for a credit card, a different minimum expense figure, or a different maximum loan term.

The result can also change by loan purpose and applicant type. A first-home buyer, investor, self-employed applicant and homeowner seeking a top-up may be assessed under different policy settings. The property, deposit, requested structure and bank's available high-DTI capacity can matter too.

A higher estimate is not automatically the better answer. The useful comparison is between lenders that suit the full application: policy fit, product features, structure, price, turnaround, conditions and your ability to manage the repayments.

Test rate, DTI and LVR: three different questions

These terms are often blended together, but each examines a different part of a home-loan application.

As at 7 September 2026, Reserve Bank DTI rules allow registered banks to make up to 20% of new owner-occupier lending above a DTI of 6 and up to 20% of new investor lending above a DTI of 7. These are portfolio speed limits, not personal approvals or absolute caps. Banks still apply their own affordability assessments, and exemptions apply to some lending.

You can pass one test and fail another. A household may have a strong deposit but not enough assessed monthly surplus, or comfortable serviceability but a DTI position outside the lender's current appetite. The final limit is usually set by whichever relevant constraint is reached first.

What can improve your assessed borrowing position?

You cannot negotiate away a lender's test rate, but you can make sure the rest of the application is accurate, efficient and matched to suitable policy.

• Reduce expensive short-term debt. Clearing a vehicle or personal loan may improve both monthly serviceability and total debt.
• Review unused credit limits. Ask how card, overdraft and buy-now-pay-later facilities will be treated before reducing or closing anything.
• Document all sustainable income. Current payslips, contracts, financial statements and a clear history can help the lender assess income correctly.
• Use a realistic budget. Sustainable changes to recurring expenses matter more than a temporary pre-application clean-up.
• Build the deposit. A larger deposit does not create income, but it reduces the loan required for the same property and improves the LVR.
• Consider timing and term carefully. Changes in employment, debt repayments or an accepted loan term can alter the result, but a longer term usually means more total interest.
• Compare suitable lender policy. Genuine differences can matter, especially for variable income, self-employment, investment property or complex ownership.

Avoid taking new credit, increasing limits or making a finance-dependent offer while your numbers are being assessed. A material change can invalidate an earlier estimate or pre-approval.

The maximum loan is not automatically your comfortable loan

A lender's maximum is the edge of its policy calculation on the information provided. Your comfortable number may be lower once you allow for rates, insurance, maintenance, childcare, travel, savings goals, career plans and unexpected costs that matter to your household.

Run your own repayment scenarios as well as the lender's assessment. Compare the proposed payment with today's budget, a higher-rate budget and the life changes you can reasonably anticipate. Then decide how much buffer you want to keep rather than treating every approved dollar as a target.