How to Stress‑Test a $2–5m Mortgage Before It Breaks You

Stress‑testing a $2–5 million mortgage in Australia means modelling at least a 3% rate rise plus a 30–50% income shock, then checking two things: whether repayments stay under roughly 30–35% of your after‑tax income, and whether you hold 6–12 months of “stress‑rate” repayments in cash or offsets. If either fails, the mortgage is too big or too fragile for your current position.

Visualising the impact of rate rises and income shocks on a large mortgage.

Step 1: Define your stress‑test assumptions

For large loans, you should be tougher than the bank.

Most Australian lenders already test you at least 3% above the actual rate due to APRA guidance (e.g. 6% actual → 9% assessment) [8,16,20]. But that doesn’t include real‑world issues like business downturns, bonuses stopping or one partner pausing work.

For $2–5m loans, a practical stress‑test is:

1. Rate shock: +3% on today’s rate.
2. Income shock: 30–50% drop in household income for 6–12 months (especially for self‑employed) [1,6,14].
3. Repayment cap: Aim to stay under ~30–35% of net income, even at the stress rate [11,18].
4. Buffer: 6–12 months of repayments at the stress rate, plus 3–6 months of essential living costs in cash/offset [5,12].

You can see how this framework plays out in more detail in /insights/stress-testing-large-eastern-suburbs-mortgage.

Step 2: Run the numbers on your $2–5m loan

Let’s use a worked example so you can copy the method this week.

Example:

• Loan: $3,000,000
• Current rate (variable, owner‑occupied): 6.0% p.a. (illustrative)
• Term: 30 years, principal & interest
• Net household income: $35,000 per month

2.1 Current and stress‑rate repayments

Indicative P&I repayments:

• At 6.0%: about $17,985/month
• At 9.0% (+3% stress rate): about $24,148/month

Now look at repayment‑to‑income ratios:

• Current: $17,985 ÷ $35,000 ≈ 51% of net income
• Stress rate: $24,148 ÷ $35,000 ≈ 69% of net income

Roy Morgan’s work on mortgage stress suggests borrowers become ‘At Risk’ once repayments exceed roughly 25–45% of after‑tax income, depending on spending patterns [2,3]. At 50–70%, this household would be deep in the danger zone.

2.2 Add an income shock

Now assume a 40% income hit (e.g. lost bonus, business downturn):

• New net income: $35,000 × 60% = $21,000/month

Ratios now:

• At 6.0%: $17,985 ÷ $21,000 ≈ 86%
• At 9.0%: $24,148 ÷ $21,000 ≈ 115% (mathematically impossible without burning savings)

Outcome: this $3m loan is not safely stress‑tested at current income.

Your job is to run the same maths on your own numbers.