Interest only vs principal and interest: which loan structure is right for Brisbane investors in 2026?
On a $630,000 investment loan, interest only repayments are currently $491 per month cheaper than principal and interest. Over five years that is $29,460 in retained cash flow. The trade-off is that your loan balance stays exactly where it started, your IO period eventually ends, and the repayments that follow are higher than they would have been. This article runs the verified numbers on both structures at three Brisbane price points so you can make the right call for your situation.
The interest only versus principal and interest question is one of the most consistently searched topics among first-time Brisbane property investors, and it receives some of the worst explanations online. Most articles either oversimplify it to "IO saves cash flow" or overcomplicate it with 30-year amortisation tables that take five minutes to read and produce no clear answer. This article does neither. It explains exactly how each structure works, runs verified 2026 calculations at three specific Brisbane price points, addresses the post-budget tax position for each structure, and gives a direct framework for which type of investor should choose which option.
The short answer, before the detail: interest only suits investors who need maximum short-term cash flow, are confident in capital growth covering the stagnant loan balance, and have a clear plan for what to do when the IO period ends. Principal and interest suits investors who want to build equity progressively, are comfortable with slightly higher monthly repayments, and are thinking about long-term wealth accumulation rather than near-term cash flow optimisation. Neither is universally better. The right answer depends on your income, your other financial obligations, your investment horizon, and your strategy for the portfolio beyond the first property.
How each structure actually works
- Lower monthly repayments during IO period
- Loan balance does not reduce during IO period
- Entire repayment is tax deductible as an expense
- IO terms typically 1 to 5 years on investment loans, up to 10 with some lenders
- Interest rate is typically 0.10% to 0.30% higher than equivalent P&I rate
- Repayments rise significantly when IO period ends and remaining term shortens
- Higher monthly repayments from day one
- Loan balance reduces progressively building equity
- Only the interest portion of each repayment is tax deductible
- Lower interest rate than IO for the same loan profile
- Repayments are stable and predictable for the full 30-year term
- Less total interest paid over the life of the loan
Current rates and the real cost difference in 2026
The rate differential between IO and P&I investment loans matters significantly because it partially offsets the cash flow saving that IO appears to offer. At April 2026, investment IO rates from major lenders sit at approximately 6.60% to 6.90% depending on LVR and lender. Investment P&I rates for the same borrower profile are approximately 6.40% to 6.70%. The rate differential is currently 0.10% to 0.30%, which is at the lower end of historical ranges. For the PropTalk calculations below, we use 5.91% for P&I and 6.19% for IO, consistent with mid-range lender pricing in June 2026.
IO investment loans carry a higher interest rate than P&I loans because lenders treat them as higher risk — the balance does not reduce during the IO period, so the lender's exposure stays constant. This rate premium partially offsets the lower monthly repayment that IO appears to offer. At 0.28% above P&I rates, the real monthly saving from IO is smaller than a surface-level comparison suggests. The calculations below use verified current-market rates to show the true difference.
| Property | Loan amount | P&I monthly (5.91%) | IO monthly (6.19%) | Monthly saving (IO) | Annual saving (IO) |
|---|---|---|---|---|---|
| Moorooka unit Purchase $787,500 | $630,000 | $3,741/mo | $3,250/mo | $491/mo | $5,893/yr |
| Brisbane unit median Purchase $876,474 | $701,179 | $4,163/mo | $3,617/mo | $547/mo | $6,558/yr |
| Oxley house Purchase $1,000,000 | $800,000 | $4,750/mo | $4,127/mo | $624/mo | $7,483/yr |
The monthly saving from IO is real and meaningful. On a Moorooka unit purchase, IO saves $491 per month or $113 per week compared to P&I at equivalent borrowing rates. Over a five-year IO period that is $29,465 in retained cash flow. The question is what happens at the end of that five years and whether the retained cash flow justifies the trade-offs.
What happens when the IO period ends
This is the section of the IO versus P&I discussion that most articles skip because the numbers are confronting. When an IO period ends the loan converts to P&I repayments on the remaining term. Because five years of the 30-year term have passed without reducing the balance, the remaining P&I repayments are calculated over 25 years on the original balance. Those repayments are higher than they would have been on a 30-year P&I loan from the start.
| Property | IO loan balance after 5 years | P&I balance after 5 years | P&I repayment (original 30yr) | P&I repayment after IO ends (25yr) | Repayment increase |
|---|---|---|---|---|---|
| Moorooka unit | $630,000 | $585,587 | $3,741/mo | $4,025/mo | +$284/mo |
| Brisbane unit median | $701,179 | $651,748 | $4,163/mo | $4,479/mo | +$316/mo |
| Oxley house | $800,000 | $743,602 | $4,750/mo | $5,110/mo | +$360/mo |
On the Moorooka unit, an investor who chose IO saves $491 per month for five years, then faces repayments of $4,025 per month for the following 25 years — $284 per month more than if they had chosen P&I from the start. The IO strategy saves cash now and costs more later. Whether that trade-off is worth it depends entirely on what the investor does with the retained cash flow during the IO period and whether their income can comfortably service the higher repayments when they arrive.
| Property | Total interest (P&I 30yr) | Total interest (IO 5yr then P&I 25yr) | Extra interest from IO strategy |
|---|---|---|---|
| Moorooka unit | $717,216 | $772,869 | +$55,653 |
| Brisbane unit median | $797,990 | $859,931 | +$61,941 |
| Oxley house | $910,082 | $980,752 | +$70,670 |
Over the full 30-year life of the loan, the IO strategy costs significantly more in total interest paid. On the Moorooka unit, choosing IO over P&I costs an additional $55,653 in interest over the life of the loan. That is the true long-term cost of the short-term cash flow saving. For an investor who uses the retained cash flow intelligently during the IO period — paying down a higher-rate debt, funding a deposit on a second property, or investing in appreciating assets — that cost can be justified. For an investor who simply spends the retained cash flow without a clear plan, the IO strategy costs them $55,653 for no strategic benefit.
The tax deductibility question
For investment properties, the interest component of both IO and P&I loan repayments is tax deductible. This is widely understood. What is less widely understood is the difference in how much of each repayment is actually deductible, and what that means for the real after-tax cost of each structure.
With an IO loan, the entire monthly repayment is interest and therefore entirely tax deductible. With a P&I loan, each repayment contains both an interest component and a principal component. Only the interest component is deductible — the principal repayment is not. In the early years of a P&I loan when the balance is high, the interest component is the majority of each repayment. But it is never 100% of the repayment as it is with IO.
| Structure | Annual repayment | Deductible interest (year 1) | Tax saving at 34.5% | Net annual cost after tax |
|---|---|---|---|---|
| Interest only (6.19%) | $38,997 | $38,997 | $13,454 | $25,543 |
| Principal and interest (5.91%) | $44,890 | $37,022 | $12,773 | $32,117 |
| Net after-tax cashflow advantage (IO) | +$681/yr | $5,211/yr better on IO |
After accounting for tax deductibility, the IO structure provides an after-tax cash flow advantage of $5,211 per year on a Moorooka unit loan, not the $5,893 gross saving the headline repayment difference suggests. The difference is smaller after tax because the P&I structure's larger interest deduction partially closes the gap. At a 39% marginal rate the after-tax advantage of IO would be slightly larger. At a 32.5% rate it would be slightly smaller.
For established properties purchased after 12 May 2026, negative gearing losses are ring-fenced to property income from 1 July 2027 and cannot be offset against wages. This does not change the deductibility of interest payments themselves — the interest expense is still fully deductible against your rental income. What changes is the treatment of the net loss if your costs exceed your rental income. The tax deductibility figures in the table above remain accurate for new build properties and for established properties purchased before 12 May 2026. Established property purchasers after that date should model their position with a registered tax agent.
Which investor should choose which structure
The IO versus P&I choice is not about which structure is objectively better. It is about which structure fits your specific financial position, your investment strategy, and your income outlook over the next five years. Here is the honest framework.
If you choose interest only, you must have a specific and credible plan for what happens when the IO period ends. The three most common plans are to refinance to a new IO period with the same or a different lender, to have paid down enough owner-occupied debt during the IO period that the higher P&I repayments are manageable, or to have built enough equity through capital growth that a refinance at a better LVR is possible. Choosing IO without a plan for what comes after is the most common and most costly mistake Brisbane property investors make with loan structure. The $284 per month increase in repayments when IO ends on a Moorooka unit is manageable if you plan for it. It is a genuine financial stress if you do not.
The one number that changes the calculation
Everything in the IO versus P&I discussion comes back to one question that most articles do not ask directly: what is your investment horizon, and do you plan to sell or hold?
If you plan to hold the property for more than ten years and eventually sell, the interest cost difference between IO and P&I is largely irrelevant compared to the capital growth you capture during that period. A Brisbane unit purchased at $787,500 today growing at 8% per year is worth approximately $1,700,000 in ten years. The $55,653 in extra interest from the IO strategy over 30 years is immaterial against a $912,500 capital gain. In this scenario IO is clearly the right choice because it maximises cash flow during the holding period and the capital growth more than compensates for the total interest cost difference.
If you plan to hold for five years and sell, the IO strategy makes even more sense because you sell before the IO period ends and never face the higher P&I repayments. You have saved $29,465 in cash flow over five years and you exit before the cost structure changes.
If you plan to hold the property as a long-term income-generating asset into retirement and eventually want it debt-free, P&I is the right structure because it systematically reduces the debt over time and gives you a clear path to owning the asset outright. IO on a property you plan to own forever means the debt never reduces unless you make principal payments voluntarily.
IO or P&I is not a question with a universal right answer. It is a question with a right answer for your specific situation, and the wrong choice costs real money over time.
Interest only saves $491 to $624 per month on Brisbane investment properties in the current rate environment, but costs $55,000 to $70,000 more in total interest over 30 years and produces significantly higher repayments when the IO period ends. Principal and interest builds equity with every payment, costs less overall, and provides a more stable and predictable long-term position. The investor who should choose IO is one with a clear strategy for the retained cash flow, a plan for when the period ends, and a portfolio expansion strategy that justifies deferring principal repayment. The investor who should choose P&I is one focused on long-term equity building, managing post-budget ring-fencing on established property, or who simply wants a predictable and sustainable repayment structure for the life of the loan. If you are not sure which applies to you, that uncertainty is the most important reason to speak to a broker before signing anything.
All repayment calculations independently verified using the standard amortisation formula at a P&I rate of 5.91% per annum and an IO rate of 6.19% per annum, consistent with mid-range lender pricing in June 2026. Stanford Financial, Interest Only vs Principal and Interest Investment Loan Guide (April 2026): investment IO rates approximately 6.60% to 6.90%, P&I rates approximately 6.40% to 6.70%, rate differential 0.10% to 0.30% at lower end of historical ranges. Money.com.au investment home loans (updated 1 June 2026): lowest P&I investment variable rate from 5.85% p.a. Finder, Best interest-only home loans (April 2026): average IO variable rate 7.27%, average P&I variable rate 6.65% owner occupier. RBA data via Savings.com.au: average new IO investment loan rate May 2025 approximately 6.0% versus P&I 5.9%. Lendology, Interest only vs P&I (April 2026): IO periods typically 1 to 5 years on investment loans. ATO tax deductibility: interest on investment loans deductible under Section 8-1 ITAA 1997. Post-budget established property treatment: Federal Budget 2026, negative gearing losses ring-fenced to property income from 1 July 2027 for established properties purchased after 12 May 2026 — interest deductibility itself is not affected, only the treatment of the net loss. Tax calculations assume 34.5% marginal rate including Medicare levy. All figures are indicative. This article does not constitute financial or tax advice. Always consult a licensed financial adviser, mortgage broker and registered tax agent before making investment decisions.