Most cash-flow trouble is bought, not developed. It is locked in on the day you commit to a price and a loan, at which point the weekly figure is largely fixed. Modelling it properly beforehand is the highest-leverage thing you can do — and it pairs naturally with buying on yield rather than price.

Why cash flow belongs in the buying decision

A property's yield tells you whether it is priced well; its cash flow tells you whether you can afford to hold it. Both matter at purchase, and they answer different questions. You can buy a well-priced property on a strong yield and still be forced to sell within two years because the weekly holding cost outran your budget the moment rates moved. Cash flow is the affordability test that keeps you in the market long enough for the investment to work.

Model at a realistic rate, not today's teaser

The single biggest input is the interest rate, and the most common mistake is modelling the introductory rate as if it lasted forever. Use the ongoing rate you will actually pay, and remember that an interest-only period ends — repayments step up when it reverts to principal and interest. A cash-flow model built on the honeymoon rate describes a property you will only own for the honeymoon.

Lenders assess your loan at an assessment rate— typically a few percent above the actual rate — precisely because they know rates move. Modelling your own cash flow at a buffer above today's rate simply applies the same prudence to your own decision.

Upfront costs vs ongoing cash flow

Two different money questions collide at purchase, and it helps to keep them apart:

Upfront costs (one-off)

Deposit, stamp duty, legal and building/pest inspection, loan fees. These decide how much you can buy and how much cash you have left as a buffer — but they do not recur.

Ongoing cash flow (every week)

Rent in, costs and loan out. This is what you live with for the whole hold, and it is the figure that determines whether the purchase is sustainable.

A frequent error is spending every dollar on the deposit and stamp duty, leaving no buffer to absorb the ongoing negative cash flow. The upfront and the ongoing need planning together.

Serviceability: what you can carry

Serviceability is the lender's word for whether your income covers your commitments with margin to spare. For your own purposes, translate it into a simple question: after this purchase, what is the total weekly out-of-pocket across every property you hold, and does your surplus income cover it comfortably — not exactly?

Add the new property's weekly cash flow to your existing portfolio's, not in isolation — negatives accumulate.
Fund the shortfall from genuine surplus income, never from the deposit savings you just spent.
Leave headroom for the property being between tenants when you least expect it.

Stress-test before you sign

A single cash-flow figure at today's rate is a snapshot in perfect conditions. Before you commit, run the property through the bad year and see whether it still holds:

Rates up 2%: how much does the weekly figure worsen, and can you still fund it?
Six weeks vacant: costs run while rent stops — is the buffer enough?
A major repair: a $5,000 unplanned cost in the first year.

The cash flow calculator runs downside, base and upside scenarios so you can see all three at once, rather than discovering them after settlement.

A pre-purchase cash-flow checklist

Achievable rent, benchmarked against comparable lets — not the agent's estimate.
Realistic vacancy allowance for the area and property type.
All operating costs, including the annual and lumpy ones.
Loan repayments at a buffered rate, with the interest-only reversion modelled.
A stress test across higher rates, longer vacancy and a major repair.
A cash buffer left over after the deposit and stamp duty, not spent on them.