KiwiSaver vs Mortgage Calculator

The question almost every Kiwi homeowner faces

You have spare cash each year. Do you throw it at the mortgage to be debt-free sooner, or invest it in KiwiSaver and let the market do the work? There is no single right answer, because the two options are not the same kind of bet. Paying down the mortgage earns you a guaranteed, tax-free return equal to your mortgage rate. Investing in KiwiSaver might earn more, but the return is uncertain and the money is locked until you turn 65 or buy a first home. This tool puts the same spare dollars into both paths over the same number of years and shows which one ends ahead on the figures you give it.

How the comparison is built

For the investing path, your annual spare cash is added at the start of each year and compounded at your KiwiSaver return after fees. For the mortgage path, the same annual amount is compounded at your mortgage interest rate, because every dollar you do not borrow saves you that rate, year after year, exactly like a guaranteed investment of the same size. Putting both on identical timing makes the comparison fair. The default run uses $6,000 a year, a 6.5 percent mortgage rate, a 6 percent KiwiSaver return, and 15 years.

$6,000 a year for 15 years

On the defaults, the mortgage path is worth about $154,524 in interest saved, while the KiwiSaver path grows to about $148,035. The mortgage wins by roughly $6,489, purely because its 6.5 percent guaranteed rate edges out the assumed 6 percent investment return. Flip the inputs so KiwiSaver returns more than the mortgage rate and the result flips too. That is the whole lesson: when your mortgage rate is higher than your expected after-fee return, paying down debt is the mathematically stronger and far safer move.

Why the matched contributions usually come first

The model treats your spare cash as a clean either-or, but real life has a free lunch you should grab before either path. If you contribute at least enough from your wages to capture the full employer contribution of 3 percent and the annual government contribution, you are getting matched money no mortgage rate can beat. The government adds 50 cents for every dollar you put in, up to $521.43 a year, once you have contributed $1,042.86. That is an instant 50 percent return on the first chunk. So the honest sequence is: contribute enough to capture the employer and government top-ups, then send any remaining spare cash to whichever path this tool says wins.

The risk and liquidity the maths hides

Two things the headline number cannot show. The mortgage return is certain and tax-free, while the KiwiSaver return is an estimate that some years will be negative. And KiwiSaver is locked until 65 or a first-home purchase, whereas a smaller mortgage gives you breathing room and, often, the ability to redraw if your bank allows it. Many New Zealanders split the difference on purpose, paying a bit extra off the loan for security while still investing for growth. There is no general capital gains tax here, so KiwiSaver growth is taxed only through the fund’s PIE rate of up to 28 percent, not as a separate gain, which slightly narrows the gap in the investment path’s favour over very long horizons.

My mortgage is fixed. Can I even make extra repayments?

Usually within limits. Most New Zealand banks let you increase repayments or make a lump sum each year on a fixed rate, often up to 5 percent of the balance, before break fees apply. Check your loan terms; if extra repayments are tightly capped, the investing path may win by default simply because you cannot deploy the cash into the mortgage.

What return should I assume for KiwiSaver?

Use an after-fee figure that matches your fund type. A growth fund might be modelled around 5 to 7 percent long term, a balanced fund a little lower, a conservative fund lower again. Subtract the fund fee before you enter the number, and stay conservative, since the mortgage rate you are comparing against is guaranteed while this is not.