When you take out an investment property loan in Australia, one of the first decisions your lender will ask you to make is whether you want an interest-only (IO) or principal and interest (P&I) loan structure. This is not a trivial choice. It directly affects your monthly repayments, your tax deductions, how quickly your debt reduces, and your overall portfolio growth strategy. The right answer depends on your tax position, cash flow situation, investment strategy, and how many properties you plan to own. This guide explains the difference, the pros and cons of each for investors, and when each structure makes sense in 2026.
Interest-Only (IO) vs Principal and Interest (P&I): The Core Difference
On a principal and interest loan, each repayment consists of two components: interest on the outstanding balance, plus a portion of the principal (the borrowed amount itself). Your debt reduces with every repayment. On a $500,000 loan at 6.5% over 30 years, your P&I repayment is approximately $3,160/month. After 5 years, you have paid down roughly $35,000 of principal, and your outstanding balance is approximately $465,000. On an interest-only loan, your repayment covers only the interest — none of the principal. Your debt does not reduce during the IO period. On the same $500,000 loan at 6.5% (IO), your monthly repayment is approximately $2,708 — around $452/month less than P&I. After 5 years, your debt is still exactly $500,000. When the IO period ends (typically after 5 years, sometimes 10), the loan reverts to P&I over the remaining term — and because the same debt must now be repaid over a shorter period, the P&I repayment after IO is higher than it would have been if you had started P&I from day one.
IO vs P&I — $500K Loan at 6.5% Comparison
IO loans have lower repayments and maintain maximum deductible interest throughout the IO period. P&I loans reduce debt and build equity faster, but result in slightly lower interest deductions each year as the principal falls. The right structure depends on your overall strategy — see the comparison below.
When Interest-Only Makes Sense for Investors
You have non-deductible home loan debt: If you have a mortgage on your own home (non-deductible) and an investment property loan (deductible), it is financially efficient to minimise the investment loan repayment (by going IO) and use the cash saving to pay down your non-deductible home loan faster. You get the same cash flow result with a better tax outcome — you are eliminating bad debt (non-deductible) while maximising good debt (deductible). You are in a high tax bracket: IO maximises deductible interest throughout the period, which is most valuable at the highest marginal rate. You want to maximise borrowing capacity for further purchases: IO repayments are lower, which can improve your assessed serviceability for subsequent investment loans. You are in a capital growth strategy: If the property’s return is primarily expected through capital gain (not rental income), IO allows you to hold with lower cash outflow while the asset appreciates.
When Principal and Interest Makes More Sense
You have no non-deductible debt: If your own home is paid off (or you don’t own one), there is no strategic benefit in keeping investment debt high. P&I reduces your debt and builds equity you can use to fund future purchases. You are late in the investment cycle: Closer to retirement, paying down debt reduces risk and eventual CGT exposure on a high equity position. IO period is expiring: When your IO term ends and you can no longer renew (APRA periodically tightens IO lending), the revert-to-P&I repayment jump can be significant — budgeting for this in advance is critical. You plan to hold long-term and eventually move in: If you might one day convert the investment property to your primary residence, building equity now is advantageous.
IO vs P&I is not a permanent decision — review your loan structure at each IO expiry and as your portfolio evolves. The optimal structure at property #1 may not be optimal by property #3, and the interplay between your home loan, investment loans, and tax position changes over time. A mortgage broker and property accountant working together can map out the most efficient structure for your specific situation.
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General Advice Warning: This article is general in nature and does not constitute personal financial advice. Please consult a licensed financial adviser before making investment decisions.
