Interest-Only Mortgages: When Are They Actually a Smart Financial Move?

A quick heads-up: The information in this article is designed as a general educational guide only and does not constitute personalised financial, tax, or legal advice. Because everyone's situation is unique (and bank policies change frequently), we recommend speaking with a licensed Financial Adviser to see how this applies to your specific circumstances. For full transparency on our terms of service, how we are remunerated by lenders, and any specific situations where a fee may apply, please read How We Get Paid.

From the moment most of us step onto the property ladder, we are given one piece of standard advice: pay down your home loan as quickly as humanly possible.

So, when you hear about people intentionally choosing an "Interest-Only" (I/O) mortgage—where their regular payments do not reduce the loan balance by a single dollar—it can sound completely backwards. Why would anyone actively choose to just tread water with their debt?

While it generally isn't the best long-term strategy for the home you live in, an interest-only structure can be a practical, strategic financial tool when applied to the right situation.

Here is a straightforward look at how interest-only mortgages work, when they can be a smart move, the scenarios where they are best avoided, and why getting your accountant involved early is essential.

Principal & Interest vs. Interest-Only: the basics

To see why an interest-only structure can be useful, it helps to compare the two ways a mortgage is repaid:

• Principal & Interest (P&I): Every repayment is split. One portion covers the bank's interest charge, and the other goes toward paying down the actual balance you borrowed (the principal). Over time, the balance steadily decreases until the home is fully paid off.
• Interest-Only (I/O): Your repayments only cover the interest charged by the lender. Because you aren't paying down the principal, your regular outgoings are significantly lower, which frees up day-to-day cash flow. However, if you borrow $600,000 on an interest-only term for three years, you will typically still owe exactly $600,000 at the end of that three-year period.

So, when does keeping your loan balance level actually make sense?

Why property investors often lean into interest-only

Managing cash flow is often the primary focus for property investors, and this is where interest-only structures are most commonly used.

By placing an investment property on an interest-only term, regular mortgage outgoings drop substantially. This often allows the rental income to comfortably cover the mortgage interest, council rates, insurance, and routine maintenance, without the investor having to top up the shortfall out of their personal salary each week.

Depending on current Inland Revenue rules and your personal tax structure, interest on investment lending can often be claimed as a tax deduction against rental earnings. Because many investors aim to build long-term wealth through capital growth rather than simply clearing the loan to zero, they often choose to redirect that preserved cash flow into other investments, home improvements, or building a deposit for their next project.

Turning your first home into a rental property

A very common scenario in New Zealand is upgrading to a new family home while keeping your original first home as a rental.

From a strategic perspective, you generally want to eliminate debt on your new, personal family home as quickly as possible, because the interest on your private home is not tax-deductible.

Homeowners in this position often look to put the rental property on an interest-only term. The surplus cash flow generated by the rental can then be redirected into making accelerated Principal & Interest repayments on their personal home.

Using interest-only as a temporary breathing space

Interest-only lending isn't solely for investors; it can also provide a temporary safety net for homeowners navigating a planned or unexpected drop in household income.

However, for a family home, this is generally best viewed as a short-term buffer rather than an ongoing plan. While it eases the pressure today, it pauses your progress on clearing your mortgage.

When an interest-only mortgage is usually the wrong move

While an interest-only facility has clear practical uses, it carries distinct risks if used in the wrong context:

• Masking true affordability: If you are buying a personal home and can only make the numbers work by using an interest-only term, you may be overextending yourself. It hides the true long-term cost of the property and leaves you vulnerable if interest rates increase.
• The lifestyle creep trap: Freeing up cash flow requires budget discipline. If the money saved each fortnight is absorbed by general lifestyle spending rather than targeted toward savings, renovations, or clearing other debt, it simply delays your financial progress.
• Property market dips: Because you are not reducing the principal balance, your equity only grows if the wider property market rises. If property values soften, you carry a greater risk of finding yourself with low or negative equity.

The hidden catch: the 25-year squeeze and payment shock

Interest-only terms do not run for the entire life of a mortgage. Banks typically approve them in blocks of 1 to 5 years.

The biggest consideration is what happens when that interest-only period expires:

1. The repayment jump

If you have a 30-year mortgage and spend the first 5 years paying interest-only, you do not get an extra 5 years added to the end. You now have just 25 years remaining to pay off the entire principal. When the loan reverts to standard Principal & Interest, your repayments will jump significantly higher than they would have been originally, because you are compressing 30 years of principal into 25 years.

2. The bank's servicing test

Because of that eventual jump in repayments, banks assess interest-only applications conservatively.

When you apply for a 5-year interest-only term, the bank’s calculator does not test your ability to afford the lower interest-only payments. Instead, they test whether your income can afford the higher repayments required over the remaining 25 years, calculated at their elevated servicing test rate. As a result, requesting an interest-only term can sometimes actually reduce the total amount a bank is willing to lend you.

Why talking to your accountant is non-negotiable

If you are considering an interest-only mortgage as part of an investment or property strategy, speaking with a qualified accountant should always be your first step.

New Zealand tax settings—including interest deductibility rules and bright-line timeframes—can be complex and are subject to change. A structure that works well for someone else may not produce the same outcome for your specific financial position. An accountant will evaluate your tax bracket, property ownership structure, and long-term intentions to confirm whether an interest-only approach genuinely supports your plans.

Structuring your mortgage to suit your goals

Deciding between Principal & Interest and an Interest-Only structure comes down to your broader financial strategy and cash flow requirements.

At Home Loan Factory, we regularly work alongside our clients' accountants and legal teams to structure lending cleanly. We can calculate what your repayments will look like across different scenarios, model the future transition back to principal repayments, and help you find a structure that works for your situation today while protecting your long-term position.

Thinking about restructuring your mortgage to free up some cash flow? Get in touch with the team at Home Loan Factory today for a clear, easygoing chat.