NZ Interest Only Mortgage Calculator

Calculate your interest-only monthly payment, see how much the payment jumps when switching to principal and interest, and compare total interest cost versus P&I from the start. Covers RBNZ investor rules and NZ tax deductibility. All figures in NZD.

$
%
yrs
IO Monthly Payment
NZ$3,495
Interest only — first 3 years
P&I Payment After IO
NZ$4,123
After IO period — 27 years remaining
Payment Jump
18.0%
NZ$628/mo increase when IO ends
Total Interest Cost
NZ$861,665
vs NZ$835,603 P&I from start (NZ$26,062 more)

Interest-only loans reduce your initial repayments but increase your total interest cost because you are not paying down the principal during the IO period.

IO Strategy Total Interest
NZ$861,665
3yr IO then 27yr P&I
P&I From Start Total Interest
NZ$835,603
30yr P&I throughout
Extra Interest from IO
NZ$26,062
The cost of choosing interest-only
IO Monthly Saving
NZ$493
Cash freed up per month during 3yr IO period
StrategyMonthly (IO Phase)Monthly (P&I Phase)Total Interest
IO for 3yr, then P&INZ$3,495NZ$4,123NZ$861,665
P&I from startNZ$3,988NZ$3,988NZ$835,603

The Reserve Bank of New Zealand (RBNZ) uses debt serviceability and loan-to-value ratio (LVR) restrictions that affect interest-only lending. Specific IO rules are set by individual banks within RBNZ's framework.

CategoryRule
InvestorsIO available up to 5 years; bank must demonstrate borrower can service P&I at end of IO period
Owner-OccupiersGenerally not available except in hardship or construction scenarios; banks scrutinise heavily
LVR (investors)RBNZ currently requires minimum 35% deposit (max 65% LVR) for most investment properties
Stress testingBanks test serviceability at a stressed rate (typically +2%) on full P&I repayments, even during IO
DSCRRental income must typically cover at least 100–125% of full P&I repayments to qualify
Refinancing IOBanks may require switch to P&I at refixing if investment thesis changes or LVR deteriorates

How to Use This NZ Interest Only Mortgage Calculator

Enter your loan amount, interest rate, interest-only period (1–5 years), and total loan term. The calculator shows your IO monthly payment, your P&I payment after the IO period ends, the payment jump percentage, and total interest cost compared to starting on P&I from day one.

Use the Advanced section to see a full IO vs P&I comparison and a payment shock table showing what happens at year 1, 2, 3, and 5 IO transitions. The Pro section covers RBNZ rules, NZ tax deductibility (fully restored April 2024), and exit strategy planning.

The Formula

IO Monthly Payment = Loan Amount × (Annual Rate ÷ 12)

P&I After IO = Loan Amount × [r(1+r)^n] ÷ [(1+r)^n − 1]
where r = monthly rate, n = remaining months after IO period

Payment Jump % = (P&I Payment − IO Payment) ÷ IO Payment × 100

Total Interest (IO strategy) = (IO Payment × IO months) + (P&I Payment × remaining months) − Loan Amount
Total Interest (P&I from start) = P&I Payment × total months − Loan Amount

The key insight: because you pay no principal during the IO period, your balance stays at the original loan amount. When P&I begins, you must repay the full balance over a shorter remaining term — producing a much higher payment than if you had started P&I from day one.

Example

Auckland Investor: $700,000 Loan, 3-Year IO

Ben purchases an Auckland rental for $1,100,000 with a 35% deposit ($385,000). He takes a 3-year IO loan on the remaining $700,000 at 7.0% on a 30-year term.

Loan Amount$700,000
Interest Rate7.0%
IO Monthly Payment$4,083
P&I After IO Period (27yr remaining)$4,842
Payment Jump+$759/mo (+18.6%)
Total Interest (IO strategy)~$1,028,000
Total Interest (P&I from start)~$982,000
Extra Interest Cost of IO~$46,000
Monthly Cash Freed During IO$759/mo × 36 months = $27,324 saved

Ben uses the $759/month saving toward a second deposit. The trade-off is $46,000 more interest over the full loan life.

Interest-Only Loans in New Zealand: Who They Are For

IO loans are primarily available to residential property investors in NZ. The RBNZ and bank policies heavily restrict IO for owner-occupiers, viewing them as risky for primary residences.

Common investor uses:

NZ Tax Deductibility Restored (April 2024)

The Government restored full mortgage interest deductibility for residential investment properties from 1 April 2024, reversing the 2021 changes. This makes IO loans more attractive for investors again because:

Always confirm your specific situation with a chartered accountant, as rules can change between tax years.

Frequently Asked Questions

Yes, primarily for investment properties. Banks will typically allow IO periods of 1–5 years for investors who meet LVR requirements (35% deposit) and can demonstrate serviceability at the full P&I payment rate. Owner-occupier IO loans are largely restricted in New Zealand and only available in specific circumstances like hardship or construction. Check with your specific bank as policies vary.
The extra interest depends on loan size, rate, and IO period length. For a $600,000 loan at 7% with a 3-year IO period on a 30-year term, you pay roughly $30,000–$45,000 more in total interest versus P&I from day one. This is because your balance does not reduce during the IO period, so you pay more interest over the remaining term. Use this calculator to see your specific extra cost.
You have options before the IO period ends: apply to refinance to a new IO period (bank reassesses you at current conditions), switch early to P&I at a lower payment by extending your term if possible, sell the property if investment goals are met, or negotiate a hardship arrangement with your bank. Banks must have assessed your ability to service P&I when you applied, so if you cannot afford the transition, contact your bank early — do not wait until you miss a payment.
Lenders assess IO loans conservatively — they test your ability to service the full P&I repayment when calculating DTI and borrowing capacity. This means an IO loan does not give you more borrowing power from a servicing perspective; banks must prove you can handle P&I even if you are currently paying IO. Some lenders add a buffer rate on top of this. Your credit score is not directly affected by choosing IO versus P&I, but missed payments on either type will negatively impact your credit history.
It depends on your strategy. Choose IO if you need cash flow to save for a second deposit and plan to hold the property long term (or sell before IO ends), and if you want to maximise tax deductions. Choose P&I if you want to build equity faster, are closer to retirement, prefer certainty in your financial position, or are risk-averse about future payment increases. Many NZ investors use IO strategically in growth phases and switch to P&I once portfolio goals are met.