The mortgage mistakes that cost investors most are about structure, not rate: cross-collateralising properties, using redraw instead of an offset, mixing personal and investment debt, running without a cash buffer, and choosing a lender that blocks your next purchase. Each one is easy to avoid when you set up the loan, and expensive to unwind later. With investor rates averaging 6.89% and new limits on negative gearing from July 2027, getting the structure right matters more than ever.
The five mistakes at a glance
| Mistake | What it can cost you | How to avoid it |
|---|---|---|
| Cross-collateralising properties | Less control when you sell or refinance; the lender can hold sale proceeds | Use a separate loan secured by each property |
| Using redraw for personal spending | Lost tax deductions on part of the loan | Hold spare cash in an offset account instead |
| Mixing home and investment debt | Paying non-deductible interest for longer | Keep separate loans; pay down the home loan first |
| No cash buffer | Forced sale if rates rise, the property is vacant or repairs hit | Hold three to six months of repayments and costs |
| Choosing a lender that limits your next step | Borrowing capacity used up, or a costly refinance | Compare credit policy, not just rate |
1. Cross-collateralising your properties
Cross-collateralisation means one loan is secured by two or more properties, or several loans are secured by the same properties. Lenders often suggest it when you use equity in your home to buy an investment, because it’s simple for them.
The problem comes later. To sell one property, the lender can revalue the others and decide how much of the sale proceeds to keep. Refinancing one loan can mean refinancing them all. A better set-up is a separate loan for the deposit, secured by your home, and a second loan secured by the investment property, ideally each with a clear purpose.
2. Using redraw instead of an offset
Redraw lets you take back extra repayments, but the ATO looks at what borrowed money is used for. If you pay extra into an investment loan and later redraw it for a car or holiday, that part of the loan becomes private debt and its interest is no longer deductible. The redraw amount is treated as new borrowing.
An offset account avoids this. Money in an offset reduces the interest charged without reducing the loan balance, so the loan’s purpose, and its deductibility, stays intact when you take the cash out.
3. Mixing home and investment debt
Interest on your home loan isn’t tax deductible; interest on an investment loan usually is, against the property’s income. So paying down the investment loan while your home loan sits untouched is usually the wrong order. Keep the loans separate, put spare cash against the home loan first, and get your structure checked by a tax adviser.
4. Running without a cash buffer
Investors on variable rates have absorbed four rises in 2026. On a $600,000 interest-only loan, the one percentage point of increases added about $6,000 a year to the interest bill. Add a vacancy, a hot water system or a special levy and a tight budget can unravel quickly.
A practical buffer is three to six months of repayments and running costs. On a $600,000 interest-only loan at 6.89% with $8,500 a year of other costs, that’s about $12,500 to $25,000 (our calculation). Keep it in an offset so it reduces interest while it waits.
From 1 July 2027 there’s another reason to plan for shortfalls. Losses on established residential property bought after 7.30 pm AEST on 12 May 2026 can only be offset against residential property income, not your salary. Don’t assume a tax refund will cover a negative cash flow on a new purchase.
5. Choosing a lender that limits your next step
Lenders differ in how they count rental income, which interest rate they use to assess your other loans, and how they treat self-employed income. The cheapest rate today can come from a lender whose policy caps how much you can borrow for the next property. APRA also limits how much new lending banks can do at six times income or more, so high-debt investors may find fewer options.
A broker who works with investors can compare policies across lenders and plan the order of your purchases. And review your rate every year or two: loyal customers are often on higher rates than new ones.
Before you sign: a quick checklist
- Each property secures its own loan, with no cross-securities.
- Your deposit loan and purchase loan are split by purpose.
- You have an offset, not just redraw, on loans you’ll hold cash against.
- You’ve stress-tested repayments at your rate plus 2%.
- You know which tax rules apply to the property, based on when you sign the contract.
For more, see our home loan tips for first-time investors.
Frequently asked questions
What is cross-collateralisation and why avoid it?
It’s when a loan is secured by more than one property, or a property secures more than one loan. It gives the lender more control: when you sell or refinance, it can revalue all the linked properties and decide how much of the proceeds to keep. Separate loans secured by individual properties keep you in control.
Why is redraw risky for investors?
Because money you redraw is treated as new borrowing for whatever you use it for. If you redraw from an investment loan for personal spending, the interest on that part is no longer tax deductible. An offset account lets you access cash without changing the loan’s purpose.
How much cash buffer should an investor hold?
Aim for three to six months of loan repayments and property running costs. On a $600,000 interest-only loan at 6.89% with $8,500 a year of other costs, that’s roughly $12,500 to $25,000. Holding it in an offset account means it saves interest while you hold it.
Sources
- Australian Taxation Office, Interest expenses on rental properties
- APRA, Macroprudential policy settings, May 2026
- Australian Government, Negative Gearing and Capital Gains Tax Reform, Budget 2026-27
- Canstar, Average home loan interest rates, as at 2 October 2026
General information only. It does not consider your objectives, financial situation or needs and is not credit, tax or financial advice.