Interest-Only vs Principal & Interest: What to Check Before Choosing

When you take out a mortgage, repayment structure is one of the choices to discuss with your lender or adviser. Principal and interest payments reduce the amount borrowed as well as paying interest. Interest-only payments cover interest for a period, leaving the loan balance unchanged unless you make separate principal repayments. The right structure depends on the contract, affordability assessment, loan purpose and repayment plan.

How Principal and Interest Works

With a principal and interest loan, each payment you make covers two components. Part goes toward reducing the amount you borrowed (the principal), and part covers the interest charged on the remaining balance. In the early years, most of your payment goes toward interest because your balance is high. As your balance decreases, the proportions shift until eventually most of each payment is reducing your principal.

This structure has a clear feature: the debt reduces over time if payments stay on track and the loan is not extended or redrawn. On a standard table loan, that can put the borrower on a path to repay the home loan by the end of the agreed term. Equity can also change with property values, fees, redraws and later borrowing decisions.

The trade-off is higher minimum monthly repayments compared with interest-only. For example, before fees and future rate changes, a $600,000 loan at 6% over 30 years would have principal-and-interest repayments of about $3,600 per month, while interest-only would be about $3,000 per month. The lower payment needs to be weighed against total interest and later repayment pressure.

The Appeal of Interest-Only

Interest-only loans attract borrowers because the minimum repayments are lower during the interest-only period. That can provide cash-flow flexibility during temporary pressure, but it does not remove the debt. For rental properties, Inland Revenue says interest may be deductible only where the interest is not private in nature and the general deductibility rules are met; excess residential rental deductions can also be ring-fenced.

Some borrowers ask about interest-only during short-term situations such as parental leave, reduced income or another temporary pressure. Consumer Protection notes that repayment changes can cost more over time because of extra interest or fees, so the lender should explain the impact and the borrower should understand the catch-up repayment path.

The appeal comes with long-term trade-offs. While you are paying interest-only, the loan balance remains static unless you make separate principal repayments. You are not building equity through repayments during that period, although market movements may still change your equity position.

The True Cost Comparison

Consider an illustrative $500,000 loan at 6% interest before fees and rate changes. On principal and interest over 30 years, total interest would be about $579,000. If the same loan stayed interest-only for 30 years, total interest would be $900,000 and the original $500,000 balance would still need to be repaid. Actual loan terms, rates and fees will change the result.

Even a temporary interest-only period can have costs. If five years of interest-only is followed by principal-and-interest repayments over the remaining 25 years, the same balance has to be repaid over less time, so repayments can rise. The lender should provide the repayment amount, interest-rate assumptions, fees and total interest so you can compare structures.

For this reason, an interest-only request should be treated as a lender-assessed loan-structure decision rather than a shortcut. Lenders must check affordability and suitability, help borrowers understand what they are signing, and assess whether repayments can be made without substantial hardship.

When Interest-Only Might Make Sense

Despite the costs, interest-only may be raised in specific circumstances. Rental-property owners may consider cash flow and interest deductibility, but tax treatment depends on IRD rules, loan purpose and apportionment. Relying on capital growth instead of principal repayment adds market risk and should not be treated as a guaranteed equity-building plan.

Short-term cash-flow challenges may also be a reason to ask the lender about options, including hardship or a repayment change. Consumer Protection says lenders may prefer to change repayments rather than risk missed payments, but a new repayment plan is likely to cost more over time. Any change should be assessed against the borrower's budget, time limits and longer-term repayment path.

Some borrowers discuss split structures, with part of the loan interest-only and part on principal and interest. This can reduce minimum repayments compared with a fully principal-and-interest structure while still reducing some debt, but the benefit depends on lender terms, fees, interest rates and whether the borrower follows a repayment plan.

Making the Right Choice

For owner-occupiers, principal and interest is often the structure to compare first because it reduces the loan balance over time. It usually costs more each month than interest-only, but it gives a clearer repayment path and avoids relying solely on future property values to improve equity.

If you are considering interest-only because principal-and-interest repayments feel too high, treat that as an affordability warning to discuss with the lender, adviser or a financial mentor. Interest-only may reduce minimum payments for a period, but it can defer the affordability problem and increase total cost.

Before changing repayment structure, compare the total amount payable, fees, interest-rate assumptions, repayment schedule after any interest-only period, and what happens if your income or property plans change. The FMA says a mortgage adviser can explain how each loan works and what it costs, and you can ask why an option is recommended and whether it is suitable for your circumstances.