How to Calculate Mortgage Repayments in Australia
Your mortgage repayment is determined by three variables: the loan amount (principal), the interest rate, and the loan term. Australian home loans are almost always structured as principal-and-interest loans — each repayment covers accrued interest first, with the remainder reducing the outstanding balance. Early in the loan, most of each payment is interest; as the balance falls, more of each payment goes toward principal.
The standard amortisation formula used by Australian lenders is:
Repayment = P × r × (1+r)^n ÷ ((1+r)^n − 1)
Where P is the loan principal, r is the periodic interest rate (annual rate ÷ number of periods per year), and n is the total number of repayments. This calculator applies this formula for monthly (n = term × 12), fortnightly (n = term × 26), and weekly (n = term × 52) frequencies.
Monthly vs Fortnightly vs Weekly Repayments
The repayment frequency you choose has a significant impact on the total interest you pay over the life of your loan — even though the weekly or fortnightly amount feels smaller.
The key is that fortnightly and weekly repayments result in more payments per year. Fortnightly repayments produce 26 payments per year — equivalent to 13 monthly payments instead of 12. That extra payment each year goes directly to reducing your principal, which reduces the interest charged in every subsequent period. Over 30 years, this compounding effect can save tens of thousands of dollars.
On a typical Australian home loan of $600,000 at 6.50% over 30 years:
- Monthly repayment: ~$3,793/month — total interest: ~$765,000
- Fortnightly repayment: ~$1,897/fortnight — total interest: ~$706,000 (save ~$59,000)
- Weekly repayment: ~$948/week — total interest: ~$703,000 (save ~$62,000)
How the RBA Cash Rate Affects Your Mortgage
The Reserve Bank of Australia (RBA) sets the official cash rate at its monthly board meetings. The cash rate is the interest rate on overnight loans between banks — it acts as a benchmark for the entire lending market. When the RBA raises the cash rate, lenders typically increase variable mortgage rates within days; when it cuts, rates follow.
Australian variable home loan rates are typically the cash rate plus a lender margin of 2–3%. Fixed rates are priced differently, based on swap market expectations for future rates rather than the current cash rate. Use this calculator to model different rate scenarios: try increasing your current rate by 1–2% to stress-test affordability, or model what a rate cut would mean for your repayments.
Offset Accounts and Redraw Facilities
Two features unique to Australian mortgages can dramatically reduce your interest bill: offset accounts and redraw facilities.
An offset account is a transaction account linked to your mortgage. The balance in your offset account reduces the principal on which interest is calculated each day. If you have a $600,000 loan and $50,000 in your offset, you only pay interest on $550,000. Over a 30-year loan, keeping $50,000 permanently in offset can save over $100,000 in interest.
A redraw facility allows you to access extra repayments you have made. Unlike an offset, redraw requires a formal request and may have fees or minimum amounts. Both features are usually available on variable loans; fixed loans typically restrict or prohibit extra repayments.
First Home Buyers: Deposits, LMI, and Government Schemes
Most Australian lenders require a minimum 5–10% deposit. Borrowing more than 80% of the property value (Loan-to-Value Ratio above 80%) triggers Lenders Mortgage Insurance (LMI) — a one-off premium that protects the lender, not you. LMI can cost $10,000–$30,000 on a typical first home purchase and is usually added to your loan balance.
The federal government's First Home Guarantee allows eligible first home buyers to purchase with a 5% deposit and no LMI, with the government guaranteeing up to 15% of the loan. State governments also offer stamp duty concessions or exemptions for first home buyers — check your state revenue office for current thresholds.
APRA (the Australian Prudential Regulation Authority) requires lenders to assess borrowers at a serviceability buffer of at least 3% above the loan rate. This means if your loan rate is 6.5%, the lender tests whether you can afford repayments at 9.5%.
Principal and Interest vs Interest Only
Most owner-occupier loans in Australia are principal-and-interest (P&I) — each repayment reduces the loan balance. Interest-only (IO) loans, common for investment properties, require only interest repayments for a set period (typically 1–5 years), after which the loan reverts to P&I. IO loans result in lower initial repayments but higher repayments once the IO period ends and do not reduce the principal balance during the IO phase. This calculator models P&I loans only.
Strategies to Pay Off Your Mortgage Faster
- Switch to fortnightly or weekly repayments. As shown above, this alone can save tens of thousands of dollars with no change to your lifestyle.
- Make extra repayments. Even $200 extra per month on a $600,000 loan can cut years off your term and save thousands in interest.
- Use an offset account. Park your savings, emergency fund, and everyday spending money in an offset to reduce your daily interest charge.
- Refinance when rates drop.If your lender's variable rate is no longer competitive, switching to a lower-rate lender can reduce your repayments and total interest immediately.
- Round up your repayments. If your calculated repayment is $3,793/month, round up to $4,000. The extra $207/month compounds into significant savings over time.