Structuring a multi‑million Eastern Suburbs mortgage: IO vs P&I

On a multi‑million‑dollar Eastern Suburbs mortgage, principal‑and‑interest (P&I) will usually be the safer default because it steadily reduces your debt and generally passes banks’ APRA‑buffered servicing tests more easily, while interest‑only (IO) maximises short‑term cashflow but costs much more total interest and carries refinancing risk if policy tightens.

The right call for a Woollahra, Bellevue Hill or Vaucluse loan is rarely “all IO” or “all P&I” – it’s a structure that matches your income volatility, tax position and 5–10 year plan.

Modelling interest-only and principal-and-interest repayments brings clarity on a large mortgage.

IO vs P&I on a $4m Eastern Suburbs home: the numbers

Let’s assume:

• Loan: $4,000,000
• Rate today: 6.5% p.a. (illustrative only)
• Term: 30 years

Scenario 1 – 5 years interest‑only, then 25 years P&I

• Years 1–5 (IO only):

  • Monthly: about $21,667
  • No principal repaid – balance still $4m after 5 years.

• Year 6–30 (principal must be cleared over 25 years):

  • Required P&I at 6.5% over 25 years: about $27,000 per month.

Total effect:

• Cashflow relief in years 1–5 vs full P&I.
• But a sharp jump of ~$5,300 per month when IO ends.
• More total interest over the life of the loan.

Scenario 2 – 30 years principal‑and‑interest from day one

• P&I at 6.5% over 30 years: about $25,300 per month.
• After 5 years you’ve cut the balance by roughly $300k–$350k.
• IO expiry cliff largely avoided.

On a $4m loan, that $21k–$27k monthly band is serious money — you want those decisions anchored to stressed numbers, not just today’s rate.

Cashflow, buffers and APRA’s 3% serviceability lens

APRA expects banks to test your borrowing at least 3% above the actual rate.

For a 6.5% loan, that means modelling at 9.5%.

A robust rule of thumb from our wider Eastern Suburbs work is:

• Total home + investment repayments, modelled as P&I at current rate +3%, should sit under ~30–35% of after‑tax income. (See /insights/design-manage-multi-million-dollar-home-loan-safely and related pieces.)

For the $4m loan example, at a stressed 9.5% over 25 years:

• P&I repayment would be roughly $34,000–$35,000 per month.

If your after‑tax household income is, say, $1.2m p.a. (~$100k per month):

• 35% of after‑tax income ≈ $35,000 per month.
• That’s right on the top of the safe band — acceptable for some, too tight for others.

This is why using IO purely to “make the servicing formula work” is dangerous. Lenders already assess most IO loans as if they were P&I over the remaining term. You can’t rely on IO to permanently boost borrowing power.