How Interest-Only Loans Work in Practice
During an interest-only period, your monthly repayment is calculated on the outstanding loan balance at the interest rate alone. No principal is repaid. On a $500,000 investment loan at 6.5% interest, the monthly repayment on an interest-only basis is approximately $2,708. The same loan on a principal and interest basis across a 30-year term produces repayments of around $3,160 per month. The difference of roughly $450 represents the principal contribution that is not being made during the interest-only window.
The outstanding loan balance does not reduce during the interest-only period. At the end of a 5-year interest-only term on a 30-year loan, the remaining 25 years must absorb the entire principal. This compresses the repayment schedule and pushes P&I repayments meaningfully higher from that point forward. That shift is something many investors do not fully model before committing to the structure.
The Cash Flow Case for Investors
The most common reason investors choose interest-only loans is to preserve monthly cash flow. Lower repayments make a negatively geared property easier to hold, particularly in markets where rental yields are below the loan rate. For investors managing capital across multiple properties, interest-only can provide operational flexibility without requiring asset sales to fund day-to-day expenses.
There is also a tax dimension worth understanding. Interest on an investment loan is generally deductible in Australia against assessable rental income and, where applicable, other income. Because interest-only repayments are entirely interest, the full monthly payment may be deductible. This is not a financial or tax recommendation, and the specific tax position for any investor depends on their income, structure, and existing deductions. A qualified accountant with property investment experience should be consulted on how this applies to individual circumstances before assuming deductibility at any particular level.
The cash flow benefit of interest-only is real, but it comes with a cost that compounds over time. Every year of the interest-only period is a year the loan balance has not reduced, which means total interest paid over the life of the loan is higher than it would be under a P&I structure from day one.
What Happens When the Interest-Only Period Ends
When an interest-only term expires, the loan automatically converts to principal and interest repayments calculated across the remaining loan term. This transition is where many investors encounter a repayment increase they had underestimated.
On a $500,000 loan with a 5-year interest-only period followed by a 25-year P&I term at 6.5%, monthly repayments increase from approximately $2,708 to $3,372, an increase of around $664 per month. If the interest rate has also risen during that period, the adjustment compounds further. Investors relying on rental income to cover repayments need to factor this transition into their cash flow modelling from the start, not just at the point the letter arrives from the lender.
At the end of an interest-only term, investors can apply to extend it for a further period, subject to a fresh serviceability assessment. Lenders are not obligated to approve an extension, and approval depends on the borrower's current income, existing debts, and the updated loan-to-value ratio of the property. In a market where property values have declined since purchase, some investors find they no longer qualify for an extension under current lending policy.
How Lenders Assess Interest-Only Applications
Interest-only loans are assessed more conservatively than standard principal and interest products. APRA requires lenders to apply a serviceability buffer of at least 3% above the loan rate. They also assess the borrower's ability to service the loan on a full P&I basis, not just the lower interest-only repayment, even though the initial repayments will be lower. This means your income needs to support the higher tested figure before the loan is approved.
For investors with multiple properties, this creates a compounding challenge. Each existing investment loan is assessed at its P&I equivalent plus the buffer, and those figures are stacked as commitments against income in subsequent applications. An investor with two existing interest-only loans may find that the tested repayment obligations from those two loans alone significantly constrain what they can borrow for a third property, even if the actual cash outflow on the existing loans is manageable.
A broker who works regularly with property investors will understand how to present a loan application to minimise these stacking effects within policy. Different lenders calculate investment loan assessments differently, and the right lender choice for an investor with existing debt can materially change the borrowing outcome.
Matching the Structure to Your Strategy
Interest-only lending is not inherently superior or inferior to principal and interest. Its suitability depends on the investor's income, tax position, portfolio stage, and intended holding period for the asset.
For investors in a growth phase, buying assets they intend to hold for 10 or more years and prioritising cash flow to expand the portfolio, interest-only can serve a clear purpose. For investors closer to retirement who are focused on reducing debt and building equity, principal and interest repayments from day one achieve that goal faster, despite the higher monthly commitment.
One useful way to frame the decision: interest-only is most defensible when the capital growth expected from the asset over the interest-only period meaningfully outpaces the equity that would have been built through principal repayments. In markets where growth is uncertain or flat, the trade-off looks less favourable. There is no universal answer, and modelling both scenarios with your accountant and broker before committing to a structure is worth the time, particularly on a purchase where the holding costs are significant.
