Formula and assumptions
For each rate, monthly payment = P × r(1+r)ⁿ ÷ ((1+r)ⁿ−1), using the entered balance and remaining term. The model assumes the rate applies for the full remaining term and excludes fees, rate changes, offsets and redraw.
Repayment buffer tool
What if your mortgage rate rises?
Compare two rates you choose, then place the modelled repayment increase beside entered household income and expenses. No “current” or forecast rate is supplied.
Rate-rise stress test
Compare two user-entered principal-and-interest rates
Entered-rate payment
$3,130 / month
Stress-rate payment
$3,892 / month
Monthly increase
$762 / month
The stressed payment is 71% of entered after-tax income remaining after entered non-mortgage expenses. This ratio is a household planning prompt, not a lender servicing test or approval threshold.
Test insurance, rates, maintenance, body-corporate costs and income shocks separately before relying on this buffer.
Save FHBG requestWorked example
For $600,000 over 30 years, an entered 4.75% rate produces about $3,130 a month. At an entered 6.75%, the same model produces about $3,892—a $762 monthly increase.
Source and verification
Deterministic FHBG amortisation model v1.0, reviewed 13 July 2026. All rates are user entered. Verify actual rates, remaining term, fees and lender servicing treatment independently.
Rate-stress questions
What rate should I use for a mortgage stress test?
Use several entered scenarios rather than treating one rate as a forecast. Include the rate you are considering, a higher rate, and any lender-provided servicing assumption you are permitted to rely on.
Is this the same test a bank uses?
No. Lenders use their own income verification, expense models, policy buffers, debt treatment and product rules. This tool only compares two principal-and-interest payments.
What expenses should I include?
Enter non-mortgage household expenses, then separately consider council rates, insurance, maintenance, body-corporate costs, dependants, transport, debt payments and irregular costs.