Compound Interest for New Zealand Savers

Summary

Compound interest is interest earned on both your original amount and on previously earned interest. For NZ savers, understanding compounding is essential for evaluating term deposits, KiwiSaver growth projections, and managed fund returns. Current NZ savings rates average around 4.12%, and most KiwiSaver funds report returns net of PIE tax.

How it works

The formula

FV = P(1 + r/n)^(nt) + PMT x [((1 + r/n)^(nt) - 1) / (r/n)]

Where P is the starting amount, r is the annual rate, n is the compounding frequency per year, t is the time in years, and PMT is the regular contribution per period.

NZ savings landscape

PIE tax advantage

Portfolio Investment Entities (PIEs) are taxed at your Prescribed Investor Rate (PIR), which is capped at 28% — lower than the top marginal income tax rate of 39%. For earners above NZ$180,000, this makes PIE funds significantly more tax-efficient than direct savings accounts taxed via RWT at their marginal rate.

KiwiSaver contributions

KiwiSaver combines employee contributions (3-10% of gross pay), employer contributions (minimum 3%), and a government contribution (up to NZ$521.43 per year if you contribute at least NZ$1,042.86). All three compound together within the fund.

Worked example

NZ$10,000 starting balance, NZ$500/month contributions, 4.12% return, 10 years, monthly compounding:

1. Monthly rate: 4.12% / 12 = 0.3433%
2. Lump-sum growth: NZ$10,000 x (1.003433)^120 = NZ$15,088
3. Annuity growth: NZ$500 x ((1.003433)^120 - 1) / 0.003433 = NZ$73,731
4. Total future value: NZ$15,088 + NZ$73,731 = NZ$88,819
5. Total contributions: NZ$10,000 + (NZ$500 x 120) = NZ$70,000
6. Interest earned: NZ$88,819 - NZ$70,000 = NZ$18,819

At a higher growth-fund rate of 7%: the same inputs produce NZ$107,298, earning NZ$37,298 in returns — demonstrating why long-term KiwiSaver investors are encouraged to choose growth funds.

Key differences from other markets

• PIE tax cap at 28% gives high earners a meaningful tax advantage on investment returns compared to the UK (where ISAs provide full tax-free growth but with contribution limits) or Australia (where super is taxed at 15%).
• KiwiSaver government contribution (up to NZ$521.43/year) acts as an automatic return booster with no equivalent in most other markets — the closest comparison is the UK’s LISA 25% bonus, which is limited to first-home purchases or age 60+.