When you take out an investment loan in Australia, one of the first decisions you’ll make is whether to use an interest-only (IO) or principal and interest (P&I) structure. It sounds like a simple loan feature — but it has real consequences for your cash flow, your tax position, and your ability to grow a portfolio.
Here’s how each works, what the trade-offs are, and how to think about which is right for your situation.
What Is an Interest-Only Loan?
With an interest-only loan, your repayments cover only the interest charged each month. You’re not paying down the loan balance (the principal). The debt stays the same throughout the IO period.
Most lenders offer IO periods of 1–5 years on investment loans, after which the loan automatically reverts to principal and interest repayments for the remaining loan term. At that point, your repayments jump — sometimes significantly — because you’re now repaying both interest and principal over a shorter remaining period.
Example: A $600,000 investment loan at 6.5% interest:
- Interest-only repayment: $3,250/month
- P&I repayment (30-year term): $3,792/month
- Difference: $542/month
That $542 difference is real cash flow — money that either stays in your pocket or goes toward reducing debt, depending on which structure you choose.
What Is a Principal and Interest Loan?
With a principal and interest loan, every repayment covers the month’s interest plus a portion of the principal. Over time, your debt reduces, your interest costs fall, and you build equity through debt repayment rather than just relying on market growth.
P&I loans typically come with slightly lower interest rates than IO loans — because the lender is getting their money back faster and sees less risk. The rate difference can be 0.1%–0.4% depending on the lender and the market environment.
The Tax Angle: Why Investors Prefer Interest-Only
Here’s the key reason so many Australian investors choose IO loans: interest is tax deductible; principal repayments are not.
Under Australian tax law, the interest you pay on a loan used to purchase an investment property is a deductible expense. The principal portion of your repayment is simply paying back borrowed money — it has no tax treatment.
This means:
- On an IO loan, 100% of every repayment is deductible.
- On a P&I loan, only the interest component is deductible — and that component shrinks every month as you pay down the principal.
For investors in the 37% or 45% tax bracket, maximising deductible interest has a meaningful impact on the after-tax cost of holding the property. An IO loan keeps the deductible amount high for the duration of the IO period.
The Cash Flow Argument for IO Loans
Beyond tax, IO loans reduce your monthly outgoing. This matters for a few reasons:
Easier to hold negatively geared properties. If your investment property costs more to hold than it earns, an IO loan reduces that shortfall. The difference between IO and P&I repayments can mean the difference between a manageable holding cost and one that puts pressure on your budget.
Preserve cash for your next purchase. If you’re trying to grow a portfolio, every dollar you’re not forced to pay in principal repayments is a dollar you can direct toward your offset account, building your next deposit. IO loans help investors move faster by freeing up capital.
Offset account strategy. Many investors pair an IO investment loan with a 100% offset account. Any surplus cash — wages, rent received — sits in the offset account and reduces the interest charged daily, without actually reducing the loan balance. This keeps the loan amount intact (preserving deductibility) while still reducing interest costs. It also keeps the cash accessible if you need it.
The Case for Principal and Interest
P&I isn’t just the “boring” option. It has real advantages in the right circumstances:
Lower interest rate. Lenders reward P&I borrowers with better rates — sometimes significantly so. In a high-rate environment, a 0.3% rate difference on a $700,000 loan saves over $2,000/year. That gap can more than offset the “benefit” of IO.
Building equity. Each P&I repayment reduces your loan balance. Over time, this builds equity you can access for future purchases — without needing the property to grow in value. For investors who want control over their equity trajectory, P&I gives them that.
Forced saving. Principal repayments are effectively forced saving. For investors who know they’d spend the extra cash flow rather than direct it toward their offset account, P&I removes that temptation.
Lower long-term interest cost. Over a 30-year loan, you pay substantially less total interest on a P&I loan because the balance is reducing continuously. IO loans maintain the full debt — and accumulate interest on the full amount — for the duration of the IO period.
What Happens When the IO Period Ends?
This is where many investors get caught off guard. When the IO period expires — usually after 5 years — the loan reverts to P&I repayments for the remaining term. Because the loan term has shortened (you’ve used 5 years of a 30-year loan), the P&I repayments are calculated over only 25 years. The result is a significant jump in monthly repayments.
Example: $600,000 IO loan at 6.5% for 5 years, then P&I for the remaining 25 years:
- IO repayment: $3,250/month
- P&I repayment after rollover (on 25-year term): $4,042/month
- Jump: $792/month
If your rental income and personal cash flow can comfortably absorb that jump, the transition is manageable. If you’re already stretched, it creates problems. Before choosing IO, make sure you’ve stress-tested what the reversion looks like.
Can You Refinance Out of IO?
Yes — and many investors do this routinely. A common strategy: take a 5-year IO loan, build equity through market growth, then refinance at the end of the IO period into a new IO loan or restructure entirely. This effectively resets the IO clock.
The risk: refinancing requires you to meet current lending criteria at the time of refinance — which may be stricter than when you first borrowed. If rates have risen, your income hasn’t grown, or your property value hasn’t moved as expected, refinancing may not be straightforward. Don’t assume a refinance will always be available.
IO vs P&I: Which Should You Choose?
There’s no universal answer, but here’s a framework:
IO generally makes more sense when:
- You’re in a high tax bracket (37%+) and maximising deductions is a priority
- The property is negatively geared and you need to manage cash flow
- You have a clear plan to deploy the cash flow savings into an offset account or next deposit
- You’re in a growth phase of portfolio building and every dollar counts
- Your home loan is P&I — it makes sense to pay down non-deductible debt first while keeping investment debt IO
P&I generally makes more sense when:
- The rate difference is significant enough to offset the cash flow benefit of IO
- The property is positively geared and the extra repayment is comfortable
- You’re in a lower tax bracket and the deductibility benefit is modest
- You’re approaching retirement and want to reduce debt rather than maintain it
- You don’t trust yourself to save the IO cash flow difference
A Note on Owner-Occupied vs Investment Debt
If you have both a home loan (non-deductible) and an investment loan (deductible), the standard advice from most accountants is to keep your home loan on P&I and aggressively pay it down, while keeping your investment loan on IO. This way, you’re eliminating the non-deductible debt first while preserving the deductible debt at its current level.
If extra cash is available, it goes into the offset account attached to your home loan — reducing the non-deductible interest without reducing the loan balance. This keeps the strategy flexible.
Talk to a Broker — This Decision Has Long-Term Implications
The IO vs P&I decision interacts with your tax position, your portfolio goals, your risk appetite, and your stage of life. The right answer in year one might not be the right answer in year five. A good mortgage broker and a property-savvy accountant should be part of this conversation before you structure any new loan.
Final Thoughts
Interest-only loans are a legitimate and widely used strategy among Australian property investors — not a financial shortcut. Used correctly, they improve cash flow, maximise tax deductions, and free up capital for portfolio growth. Used without planning, they can leave you exposed when the IO period ends.
Know what you’re signing up for. Model the numbers at reversion. And make sure the structure fits your actual goals — not just what sounds appealing on paper.
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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.