Lending Guide
Most New Zealanders overpay by hundreds of thousands in interest simply because they never review their mortgage. These strategies — applied early — can save you years and tens of thousands of dollars.
In this guide
When you take out a 30-year mortgage, the bank calculates repayments so you pay just enough interest each period while slowly reducing the principal. In the early years, the vast majority of each payment goes straight to interest — and almost nothing reduces what you owe.
On a $550,000 loan at 6.5% over 30 years, your monthly repayment is approximately $3,476. In month one, $2,979 of that goes to interest and only $497 pays down your principal. After 5 full years of payments, you've made 60 payments totalling $208,560 — but your remaining balance is still approximately $516,000. You've barely moved the needle on the principal.
On a $550,000 loan at 6.5% over 30 years:
| Metric | Amount |
|---|---|
| Original loan | $550,000 |
| Total repaid over 30 years | $1,251,360 |
| Total interest paid | $701,360 |
| Interest as % of total cost | 56% |
You pay more in interest than the original price of your home. Every dollar you put in early reduces the principal that future interest is calculated on — compounding the saving over the remaining term.
The single easiest change — and it costs you nothing extra
There are 52 weeks in a year, which means 26 fortnights — not 24. When you pay half your monthly amount every two weeks, you make the equivalent of 13 monthly payments per year instead of 12. That one extra monthly payment per year, applied consistently, has a dramatic effect over a 25–30 year loan.
This isn't a trick or a financial product — it's pure maths. The extra payment goes entirely to principal reduction, cutting every future interest calculation for the rest of the loan.
| Payment method | Repayment | Total interest | Loan paid off |
|---|---|---|---|
| Monthly | $3,476/month | $701,360 | 30 years |
| Fortnightly | $1,738/fortnight | ~$629,000 | ~26.5 years |
Small consistent amounts compound significantly over decades
Adding a fixed extra amount to every fortnightly repayment is one of the most powerful strategies available. Because every dollar goes directly to reducing principal, it reduces the balance on which future interest is calculated — creating a compounding saving effect that grows throughout the loan term.
The earlier in the loan you start, the greater the impact. An extra $200/fortnight in year one saves significantly more than $200/fortnight starting in year ten — because the interest reduction compounds across more remaining years.
| Extra per fortnight | Interest saved | Years saved | Loan term |
|---|---|---|---|
| $0 (base fortnightly) | — | — | 26.5 yrs |
| +$100/fortnight | ~$82,000 | ~3 years | 23.5 yrs |
| +$200/fortnight | ~$148,000 | ~5 years | 21.5 yrs |
| +$500/fortnight | ~$270,000 | ~9 years | 17.5 yrs |
Savings are calculated from the fortnightly base (after switching from monthly). Figures are indicative based on standard table mortgage amortisation at 6.5% p.a. Use our repayment calculator to model your specific numbers.
Bonuses, inheritances, and windfalls — put them to work
A lump sum applied to your mortgage principal cuts every future interest calculation instantly. Unlike a term deposit or savings account where you earn interest on the amount, your mortgage is charging you interest on the balance — so every dollar you remove from that balance saves you its interest cost compounded across the remaining term.
A single $30,000 lump sum on a $550,000 loan early in the term can save over $120,000 in total interest over the life of the loan — because that $30,000 never attracts interest again, and the earlier you apply it, the longer the compounding saving runs.
The trap most borrowers fall into — and how to avoid it
When your fixed rate term ends and you refix at a lower rate, your bank will often recalculate your minimum repayment downward. It's tempting to accept the lower payment — you're spending less each month, so it feels like a win. It isn't.
The correct move is to keep your repayment at the same dollar amount as before — or higher. The extra amount above the new minimum goes entirely to principal, dramatically accelerating your payoff schedule. You've already proven you can afford the higher repayment, and now more of it is working for you instead of going to interest.
$550,000 mortgage, 25 years remaining:
| Scenario | Repayment | Total interest | Loan paid off |
|---|---|---|---|
| Reduce to new minimum at 6% | $3,542/mth | $512,600 | 25 years |
| Keep repayment at 7% level | $3,884/mth | $357,200 | ~20 years |
Every expiry is an opportunity — don't let it pass by default
Most people set up a mortgage, fix it for a term, and then simply refix at whatever the bank offers when it expires. A refix without a full review is a missed opportunity. At every expiry date, you should be asking:
Which term should I fix for?
Where are rates heading? Is a 1-year or 2-year fix better right now? Should I split across multiple terms?
Is my split still right?
Do I have enough floating for flexibility? Too much floating and I'm paying a higher rate. Too little and I can't make extra repayments.
Could I refinance to a better lender?
Rate expiry is the best time to switch banks without break fees. We compare all lenders every time — sometimes a cashback offer plus a better rate makes switching worthwhile.
Can I increase my repayment?
If your income has grown since you set up the loan, this is the moment to increase your regular payment and bank the long-term saving.
We contact every Trebla client before their fixed rate expires — so you never miss the window. Register with us now →
Make every dollar in your account work against your mortgage
For disciplined borrowers, offset and revolving credit structures can reduce interest significantly without requiring any extra payments. Instead of earning taxable interest in a savings account (at 4–5%), your savings sit against your mortgage balance — saving you interest at your mortgage rate (6%+), tax-free.
Offset mortgage
Your savings accounts are linked to your mortgage. Interest is only charged on the difference. $40,000 in savings against a $550,000 mortgage = interest on $510,000 only. Your savings remain accessible at all times.
Revolving credit
Your mortgage and everyday banking merge into one account. Any money sitting in the account reduces your daily interest charge. Your salary credited on payday reduces your balance instantly — even if only for a few days.
For a full breakdown of both structures, see our Mortgage Types Guide →
Actions you can take right now — ranked by impact
Switch to fortnightly repayments today
Costs nothing. Saves tens of thousands. Call your bank or ask us to arrange it — takes 10 minutes.
Set up an automatic extra $100–$200 per fortnight
Automate it so it happens without thinking. Even $100 extra per fortnight saves ~$82,000 over the life of a $550,000 loan.
Note your next fixed rate expiry date — and diarise it
Put it in your calendar 6 weeks before expiry. That's your window to review, compare lenders, and refix without defaulting to floating. Or register with us and we'll do it for you.
When you refix at a lower rate — keep the repayment the same
Do not reduce your payment when rates fall. The saving goes to principal. This one decision alone can save 4–5 years off your loan.
Apply every windfall to your mortgage principal
Tax refund, bonus, inheritance — before you decide what to do with it, calculate the mortgage saving. At 6.5%, a $20,000 lump sum saves more than it would earn in most savings accounts, tax-free.
Keep a floating portion of 20–30% for full flexibility
This portion has no break fees and no extra repayment limits. Direct all extra payments here. Fix the rest for a competitive rate.
Get a full mortgage review every 12–24 months
Your situation changes. Interest rates change. New products become available. A review costs nothing and regularly saves clients thousands they didn't know they were leaving behind.
All figures are indicative and based on a standard table mortgage. Actual savings depend on your loan balance, interest rate, term, and lender policies. This guide is general in nature and is not financial advice. Read our disclosure statement →
