Interest-Only Loans: When They Help Investors and When They Trap

Interest-only (IO) lending sits in a strange place. It is either a tax-efficient cash flow tool or a slow-motion trap, depending almost entirely on who is holding the loan and why. The mechanics are the same for everyone. The outcomes are not.

Here is how to tell which camp you are in.

The cash flow arithmetic that actually matters

Take a $600,000 loan at 6.5%. Principal and interest (P&I) over 30 years costs roughly $3,792 a month. Interest-only on the same loan costs $3,250 a month. That $542 difference is what most people focus on, but the number that changes your strategy is what happens to deductibility.

In Australia, interest on funds borrowed to produce assessable income is deductible. The principal component of a P&I repayment is not. So on that $600,000 loan, the entire $3,250 IO payment is deductible, while under P&I only the interest slice (starting around $3,250 and falling each month as the balance drops) qualifies. Every dollar of principal you pay down is a dollar you cannot deduct.

For a negatively geared investor on the 45% marginal bracket plus Medicare levy, that distinction is worth real money. IO keeps the deductible portion of the payment as high as possible for as long as the term runs.

The UK story is different and worth stating plainly. Since the Section 24 changes fully bit in 2020, individual landlords no longer deduct mortgage interest from rental income at all. Instead they get a 20% tax credit on finance costs. A higher-rate or additional-rate taxpayer therefore gets far less relief on IO interest than an Australian investor does. This is why many UK landlords now hold property through a limited company, where interest remains a genuine business expense. If you are a UK investor weighing IO, the vehicle matters more than the loan structure.

New Zealand removed interest deductibility for most residential rentals and has since been phasing it back in, with 100% deductibility restored from the 2025/26 year for existing properties. IO in NZ is therefore a cash flow decision again rather than a purely tax-driven one.

Where IO genuinely fits

IO makes sense when the borrowed money is doing something more productive than sitting in your loan balance.

• You are accumulating deposits for the next purchase. The $542 a month you are not paying in principal becomes deposit capital. If you can redeploy it into an asset compounding faster than your loan rate after tax, holding IO is rational.
• You are running an offset strategy. Money in an offset account against an IO loan reduces interest identically to paying down principal, but stays liquid and does not lose you deductibility. This is the cleanest case for IO in Australia.
• You have a defined holding period and an exit. If you plan to sell in five years, principal reduction barely moves your equity position. Capital growth does the work.
• DSCR is tight during a construction or renovation window. Lower payments buy you breathing room until rent stabilises.

Where IO becomes a problem

The trap is the IO period ending. A 5-year IO term on a 30-year loan means the P&I repayments afterward are amortised over 25 years, not 30. On that $600,000 loan, the repayment jumps from $3,250 to roughly $4,050 a month, a 25% increase overnight. If your servicing was already stretched, that reset can force a sale at the wrong point in the cycle.

IO is also poor for investors who:

• Are near retirement and want debt reduced before income stops
• Hold a single property with no acquisition pipeline, where the deposit-building rationale disappears
• Lack the discipline to keep the offset funded, meaning the money simply gets spent

Model the reset before you sign

The mistake is looking at the IO payment in isolation. What you need is the full picture: the DSCR during the IO period, the DSCR after the reset, and how both interact with your other holdings and your projected income.

This is exactly the kind of scenario worth modelling before you commit rather than discovering at year five. Run the acquisition with an IO period, then project the cash flow across the full holding term including the reset, and check the portfolio-wide DSCR at the point the repayment jumps. If the numbers only work while the loan is IO, the loan is not solving a problem. It is deferring one.

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