You Can't Undo a Bad Acquisition. Model It First.

The problem with buying a fourth property is that your first three don’t stand still while you decide. A new mortgage shifts your total debt-to-income ratio. The stamp duty and legals drain the cash buffer you were relying on for the next vacancy. The rent might cover the new loan on paper, but your combined DTI — total debt divided by gross income — could quietly move past the point where your next refinance or acquisition gets declined.

Most people work this out in a spreadsheet built for one property, then find out the real answer six months after settlement. Pre-purchase scenario modelling is there so you see the whole-portfolio consequence before you sign.

Model the acquisition against what you already own

The point of modelling inside a portfolio tool rather than a standalone calculator is that the new property doesn’t exist in isolation. You add a candidate purchase as a scenario, enter the numbers you’d enter for a real property (purchase price, deposit, loan, rate, rent, outgoings), and the dashboard recalculates as if you already owned it.

So instead of “this property yields 4.6% gross,” you get the version that actually matters: your blended portfolio LVR moves from 58% to 64%, your total debt-to-income ratio climbs from 4.8x to 5.6x (approaching the APRA 6x cap where lenders start to constrain new lending), and your aggregate monthly cash flow drops from +$1,240 to +$310.

Those are the numbers you can’t get from a single-property calculator, because they depend on the three loans, four rents, and two offset accounts you already have running.

A concrete example

Say you own two properties in Brisbane and one in Auckland, and you’re weighing a $780,000 unit in Melbourne.

You model it at an 80% loan ($624,000), 6.3% interest-only, with acquisition costs of approximately $57,000 — stamp duty in Victoria on a $780,000 investment property runs around $42,000 (calculated at $3,070 plus 6% on the dutiable value above $130,000), plus buyer’s agent fees and legals. The scenario shows your cash buffer dropping from $61,000 to around $4,000 after settlement.

That’s not just tight — it flags you’re left with almost no buffer going into the first vacancy. The scenario also shows your Auckland property’s fixed rate rolling off in eight months, and re-fixing at current rates would cost roughly $180 more a month.

Now the Melbourne purchase looks different. Not necessarily the wrong call, but a different decision. You can then model the same purchase at 70% LVR to reduce the stamp duty hit proportionally and see the buffer comparison, or push the settlement timeline out, and compare both side by side before you commit to either.

Run more than one version

A single scenario tells you whether one specific deal works. The more useful exercise is running the same property under different assumptions.

Model the Melbourne unit at 6.3% and again at 7.5% to see how a rate rise erodes the position. Model it with six weeks of vacancy in year one instead of zero. Model it as interest-only versus principal and interest to see the cash flow difference now against the equity difference over a five-year hold.

Because the forecasting runs across your chosen holding period, you’re not just checking whether month one works. You’re checking whether the position holds up when the depreciation schedule tapers, when the fixed rate expires, and when you’d realistically be looking to refinance or sell.

One additional variable for post-2027 acquisitions: the 2026-27 Budget confirmed that negative gearing against salary income will be restricted for new established-property purchases from 1 July 2027. If the Melbourne purchase happens after that date and it’s an established property, the tax treatment of annual holding losses changes. Your scenario model should include an after-tax cash flow view that reflects which negative gearing rules will actually apply.

Structure matters before you buy, not after

If you’re buying through an SMSF or an SPV rather than in your own name, the ownership structure changes the numbers meaningfully — and it’s expensive to fix after settlement.

You can set the scenario’s owner to the trust or SMSF and see how the deal sits against that entity’s existing holdings and borrowing, separate from your personal portfolio. If the SMSF already holds one property with a limited recourse loan, modelling the second acquisition against that specific entity tells you whether the fund’s cash flow supports it, rather than lumping everything into one undifferentiated total.

When the scenario becomes real

The work you put into a scenario isn’t thrown away when you decide to proceed. Once you commit, the modelled property converts into a tracked property, and the assumptions you entered become the forecast you record actuals against. So the rent and outgoings you estimated during modelling are the baseline you check reality against once the property is live and tenanted.

That closes the loop most spreadsheets never do: you find out whether the deal you modelled is the deal you actually got.

Scenario modelling is available on the free tier for your first two properties, so you can model model acquisition before deciding whether it’s worth committing capital.

Track your own portfolio free → prop-insights.app