What Happens When Interest Rates Move — and Why You Should Care Now

When you're focused on saving a deposit and finding a house, interest rates feel like background noise. They shouldn't. The rate you lock in determines how much you can borrow, what you'll pay each month, and whether you can actually afford the property you've set your heart on.

The maths most people skip

A small rate change has an outsized impact. Here's what a $500,000 loan over 25 years looks like at different rates:

• 5.5%: $3,070/month
• 6.5%: $3,377/month (+$307)
• 7.5%: $3,695/month (+$625 vs 5.5%)
• 8.5%: $4,023/month (+$953 vs 5.5%)

The difference between 5.5% and 8.5% is nearly $1,000 per month — or $12,000 per year. Over the life of the loan, that's over $170,000 in extra interest. And because banks assess your borrowing power at a stress-test rate (currently 7%), a rise in actual rates doesn't just cost you more — it can shrink the amount you're allowed to borrow in the first place.

Your "breaking point" rate

Every borrower has a rate at which their mortgage becomes unaffordable. Not "uncomfortable" — genuinely unaffordable, where monthly expenses exceed monthly income. Knowing this number is critical, but almost nobody calculates it before they buy.

Your breaking point depends on your specific financial situation: your net income, your living expenses, your other debts, and the size of your mortgage. Two households borrowing the same amount can have very different breaking points based on their expense profiles.

If your breaking point is 8% and rates are currently 6.5%, you have a 1.5% buffer. That might sound like a lot — until you remember that NZ mortgage rates hit 6.7% in late 2023 and were above 8% during parts of 2008. Rate movements of 1–2% happen regularly over a mortgage's lifetime.

Fixed vs. floating — the Kiwi dilemma

Most NZ borrowers fix their rate for 1–3 years. This provides certainty for that period, but creates a re-fixing event every 1–3 years where your payments could jump significantly. If you fixed at 5% and rates have risen to 7% by the time you refix, your monthly payment increases by roughly $300–$400 on a $500,000 loan.

The uncertainty makes planning difficult. Do you fix for longer at a higher rate for security? Or fix short at a lower rate and hope rates stay down? Split your loan across multiple terms? Every option involves a trade-off, and most first-home buyers make this decision with very little information about their own financial resilience.

Why this matters before you buy

The time to stress-test your mortgage is before you commit to a purchase price — not after. If you buy at the top of your borrowing capacity at today's rates, you have zero buffer for rate increases. If rates rise 1% at your next re-fix, you could go from comfortable to stressed overnight.

Understanding your rate sensitivity before you buy lets you make smarter decisions:

• Buy at a price that leaves a buffer for rate increases
• Choose a loan term and structure that fits your risk tolerance
• Know exactly when your finances would become stressed — so you can plan, not panic

MortgageReady's Rate Sensitivity Explorer

The Rate Sensitivity Explorer in MortgageReady lets you drag a slider between 4% and 10% and watch your borrowing power, monthly payments, and surplus update in real time. It identifies your personal breaking point — the exact rate where your monthly budget turns negative.

It also shows a full table of how your four borrowing scenarios (conservative, comfortable, optimistic, and your target property) change at each rate. You can adjust the loan term too, so you can see the difference between a 25-year and a 30-year mortgage at any rate.

Instead of guessing whether you could handle a rate rise, you get a concrete answer based on your actual income, expenses, and debts.

Try it free — see your breaking point and plan your purchase with confidence.