Investment Loan Optimisation: What Not to Overlook

Most plumbers with an investment property are paying more than they need to and claiming less than they could.

Optimising an investment loan means setting it up so you pay the least amount of non-deductible interest, keep rental income flowing tax-efficiently, and position yourself to add more properties without blowing out your debt. It's not about chasing the lowest advertised rate. It's about the loan structure, the features you actually use, and how the debt sits alongside your owner-occupied borrowing.

Why Loan Structure Matters More Than Rate

A plumber earning solid income with a work ute loan, an owner-occupied mortgage, and one investment property will often have all three debts sitting in separate silos. The investment loan might be on a slightly higher rate because it was taken out a few years ago, and the plumber is making extra repayments into the offset on the owner-occupied loan without realising those funds could be working harder elsewhere.

Consider a plumber who owns a unit in Blacktown generating rental income of around $2,200 per month. The loan balance is sitting at $420,000 on a variable rate with no offset account. Meanwhile, the plumber's owner-occupied loan has $35,000 sitting in an offset account, effectively earning nothing in tax benefit because owner-occupied interest isn't deductible. If that $35,000 were moved into an offset against the investment loan instead, it would reduce the taxable investment debt and lower the monthly interest bill on borrowing that can't be claimed. That's the difference between tax-deductible interest on $420,000 and tax-deductible interest on $385,000. Over a year, that's real money left on the table.

Splitting Loans to Protect Deductibility

When you take cash out of an investment loan for something unrelated to the property, you dilute the deduction. If you redraw funds to buy a ute or renovate your own bathroom, that portion of the loan is no longer deductible, and untangling it later is a headache.

The solution is to split the investment loan into separate accounts from the start. One account covers the property purchase. Another covers any future renovation or capital works on that property. If you need to access equity for something else, you draw it from a third split that you knowingly tag as non-deductible. Most lenders allow multiple splits under the one loan without charging extra, and it keeps your maximise tax deductions claim rock-solid if the ATO ever asks questions.

In our experience, plumbers who plan to expand their property portfolio benefit most from this approach because each property and each purpose gets its own loan account. When it's time to refinance or release equity, you're not stuck trying to reconstruct how much of a $600,000 loan balance relates to what.

Interest-Only Repayments and Cash Flow

Interest-only periods make sense when rental income only just covers holding costs, or when you want to redirect cash toward paying down non-deductible debt or buying the next property. You're not building equity through repayments, but you're keeping the full loan balance deductible and freeing up monthly cash flow.

For plumbers who run their own business, that cash flow can be the difference between taking on another apprentice or delaying growth. An interest-only period typically runs for five years, and you can often negotiate another five-year term when it expires, depending on the lender and the loan-to-value ratio at the time. Once the non-deductible debt is cleared or the next property is locked in, you can flip back to principal and interest and start reducing the balance.

Using Equity Without Refinancing the Whole Loan

When property values rise, so does your usable equity. But pulling that equity out doesn't mean you have to refinance your entire investment loan and cop break costs if you're on a fixed rate, or lose a discounted variable rate you negotiated years ago.

Some lenders let you add a new split to an existing loan to access equity, keeping the original loan untouched. Others offer a top-up facility that sits separately but links to the same security. If neither option is available with your current lender, a second mortgage with a different lender over the same property can work, though it adds a layer of complexity when you eventually sell.

The key is to avoid a scenario where you refinance a $400,000 loan at a discounted rate just to pull out $80,000 in equity, and end up with a new $480,000 loan at a higher rate with worse features. That's a common mistake, and it's completely avoidable if you know what to ask for before you apply. You can read more about this in our guide on equity release loans.

Fixed Versus Variable for Investment Properties

Variable rates on investment loans give you full offset access and unlimited extra repayments, which makes them the default choice for most plumbers. Fixed rates lock in repayments for a set period, but they usually don't come with an offset, and extra repayments are capped at around $10,000 to $30,000 per year depending on the lender.

If you're holding the property long-term and want certainty on repayments, fixing a portion of the loan can work. But if you plan to sell, refinance, or access equity in the next few years, a fixed rate will slug you with break costs if rates have dropped since you locked in. For that reason, a 50/50 split between fixed and variable is often the middle ground: some rate protection, some flexibility, and less exposure to break costs if plans change.

Impact of the Budget Changes on Loan Strategy

From 1 July 2027, losses from established residential properties bought after 12 May 2026 can only be offset against rental income or capital gains from residential property, not against wage income. If you bought your investment property before Budget night in May this year, the old rules still apply and you can continue to claim losses against your plumbing income.

If you're looking to add another property and you buy an established dwelling after that date, your deductions are quarantined. That doesn't mean the deductions disappear, but it does mean they won't reduce your tax bill in the year you make the loss unless you have other rental income to offset them against. New builds remain exempt, so they still qualify for full deductions and the option to choose between the old or new capital gains treatment when you sell.

This changes the math on holding costs. A plumber who previously relied on negative gearing to reduce taxable income by $15,000 a year will need to plan for a higher tax bill if the next property is an established dwelling, or consider new builds and off-the-plan developments as a way to keep the tax benefit intact. It also makes loan structure more important, because you want every dollar of deductible interest genuinely tied to income-producing debt.

Offset Accounts on Investment Loans

An offset account linked to your investment loan reduces the balance on which interest is calculated, but it also reduces the amount of interest you can claim as a deduction. That sounds like a bad thing, but it's not.

If you're holding $40,000 in an offset against a $500,000 investment loan, you're only paying interest on $460,000, and you can only claim interest on $460,000. But you're also not paying tax on the interest you've saved, because the offset has done its job. The result is you've reduced your out-of-pocket interest cost without losing the deduction where it counts.

Where it gets messy is when you use the offset to park business income, pay bills, and then top it back up again. Every deposit and withdrawal creates a separate transaction, and if you're ever audited, you need to show the funds in the offset were genuinely savings, not a revolving door of business expenses. Keeping the investment offset separate from your everyday banking makes this much easier to manage.

When to Refinance and When to Leave It Alone

Refinancing an investment loan makes sense when you're paying a rate that's more than 0.50% above what new customers are getting from the same lender, or when your current loan doesn't have the features you need. It doesn't make sense when the rate difference is marginal and you'll be up for application fees, valuation costs, and discharge fees that eat up any saving within the first year.

A good rule is to compare the total cost of staying versus switching over a two-year period, not just the monthly repayment difference. Include all the fees, factor in any break costs if you're on a fixed rate, and check whether the new lender's offset and redraw policies actually match how you use the loan. You can explore your options in more detail through our investment loan refinancing page.

If you've been with the same lender for more than three years and haven't asked for a rate review, that's the first step before considering a refinance. Most lenders will negotiate to keep you, and if they won't, that tells you everything you need to know.

Optimising your investment loan means making sure the structure, the features, and the repayment type all line up with what you're actually trying to do. If you're not sure whether your current setup is working for you or costing you, call one of our team or book an appointment at a time that works for you. We'll go through your loans, your plans, and the numbers, and we'll tell you straight whether a change makes sense or whether you're already where you need to be.

Frequently Asked Questions