Most Kiwi borrowers don't fix their whole mortgage at one rate for one term. Here's why splitting it up is standard practice — and how to think about it.
Ask most New Zealand mortgage holders what interest rate they're on, and they'll often give you two or three different answers for the one mortgage. That's because in NZ, it's standard to split a single mortgage into several sub-loans, each fixed for a different term, rather than fixing the entire balance at once. If you're new to this, it can look unnecessarily complicated — but the logic behind it is straightforward risk management.
Fixing your entire mortgage on one rate for one term is a bet that rates will move in your favour, or at least stay flat, by the time that term ends. If rates have risen sharply when your fixed term expires, your whole mortgage repriced at once, which can mean a large jump in repayments overnight. Splitting the loan into portions with staggered expiry dates spreads that risk: only part of your mortgage repricing at any given time softens the impact of a bad-timing rate rise, and gives you the flexibility to make extra repayments on the sooner-maturing portions.
A common structure looks something like this: a large chunk (say 60–70% of the loan) fixed for a longer term of 18–24 months where you expect more rate certainty, a smaller chunk fixed for 6–12 months to take advantage of shorter-term rates or upcoming refixing flexibility, and sometimes a small floating portion that allows unlimited extra repayments without break fees. There's no single "correct" split — it depends on your risk tolerance, how much certainty you want in your budgeting, and your view (or your broker's view) on where interest rates are headed.
Within each sub-loan, you also choose between interest-only (IO) and principal & interest (P&I) repayments. IO payments only cover the interest charged — your balance doesn't reduce — and are typically used short-term by investors or by owner-occupiers temporarily managing cash flow. P&I payments include a portion that reduces the principal, gradually paying off the loan over its amortisation period (commonly 25–30 years). Most owner-occupier lending is P&I; a mortgage sitting entirely on interest-only for its full term will never actually be repaid.
When a fixed term on one of your sub-loans expires, that portion automatically rolls onto your bank's floating rate unless you actively refix it — usually to a new fixed term at the bank's current rate, though you're also free to switch lenders at this point. This is the moment most borrowers review whether to change their split, add an extra repayment lump sum without break-fee penalties (many banks allow this at refix), or renegotiate for a better rate. Missing your refix date and drifting onto the floating rate, even briefly, can be expensive, since floating rates are typically the highest rate a bank offers.
On a principal & interest loan, even modest recurring extra repayments compound significantly over a 25–30 year term, because every extra dollar of principal paid down early stops accruing interest for the rest of the loan's life. An extra $100 a fortnight on a large mortgage can shave years off the payoff date and save tens of thousands in total interest — the earlier in the loan term the extra repayments start, the bigger the effect, since more of the loan's life remains for the saved interest to compound away.
Because each sub-loan can sit on a different rate, your true cost of borrowing is the amount-weighted average across all portions — not whichever rate you remember most easily. This is the number worth tracking when comparing your mortgage to what's currently on offer elsewhere.
The right split is specific to your loan size, risk appetite, and repayment goals — which is exactly the kind of thing that's hard to eyeball and easy to model. Running a few scenarios side by side (a heavier long-fix split vs a heavier short-fix split, with and without extra repayments) usually makes the trade-offs obvious in a way that a single "what rate should I get" conversation doesn't.
If you're weighing a mortgage against buying an investment property instead, our rental yield & cash flow guide covers how the numbers differ once rent and property expenses enter the picture.