Why Your Mortgage Interest Repayments Change Each Month in Australia

If you’ve ever noticed that your mortgage interest repayments vary slightly from month to month, you’re not alone. Many borrowers are surprised to see fluctuations, especially when they believe they’re paying a consistent monthly amount. In reality, mortgage interest in Australia is calculated daily and paid monthly, and this is the key to understanding the changes.

Let’s break down why this happens and what factors influence your repayments.

1. Interest is Calculated Daily, Paid Monthly

In Australia, most lenders calculate interest on a daily balance and charge it monthly in arrears. This means:

• Every day, the lender applies the interest rate to the outstanding loan balance.

• The total interest for each day in the month is then added up.

• That total becomes the interest portion of your repayment for the month.

So if your balance is $500,000 and your interest rate is 6.00% per annum, the daily interest is:

$500,000 x (6% / 365) = $82.19 per day

Multiply that by the number of days in the month and you get the interest charged that month.

2. The Number of Days in the Month Matters

Because interest is calculated per day, months with more days result in higher interest charged.

For example:

• February (28 days) = 28 × daily interest

• March (31 days) = 31 × daily interest

So, even with the same interest rate and loan balance, your repayment will be higher in March than in February.

3. Changes in Your Interest Rate

If your interest rate changes during the month — for example, due to an RBA cash rate movement — the lender will apply:

• The old rate for the days before the change, and

• The new rate for the days after the change.

The interest charged will reflect this combination. You may not see the full impact of a rate cut or rise until the following month.

4. Repayment Frequency and Timing

Even if you pay weekly or fortnightly, the lender still calculates interest daily and reconciles the account monthly.

Also, the exact date when a repayment is made affects the interest total. If a payment is slightly delayed or early, that affects how many days interest accrues on a higher balance.

5. Loan Balance Changes

If you make:

• Extra repayments, your loan balance drops, so future daily interest drops.

• Withdrawals (e.g., from a redraw facility), your balance increases, so interest goes up.

Even minor changes to the balance can impact the interest charged over the month.

So, Why Did My Repayment Drop This Month?

If your monthly repayment amount has changed, it could be due to:

• A reduction in interest rates (partially or fully reflected depending on the timing)

• Fewer days in the month

• A lower outstanding balance

• A change in the repayment schedule (e.g. end of fixed rate, switch to interest-only, etc.)

Most lenders are quite precise in their calculations. If you've been with the lender for a while and repayments have generally been correct, it’s likely the recent changes are normal and expected.

What If It Looks Wrong?

If you suspect the calculation is incorrect:

1. Review your repayment history and the interest rate applied.

2. Calculate the daily interest manually, using your balance and rate.

3. Contact your lender and ask for a breakdown of how the interest was calculated for that month. Most lenders can provide this easily.

Final Thoughts

While mortgage repayments might look fixed, the interest component behind the scenes is always moving. Understanding daily interest calculations helps you make sense of changes and manage your loan more effectively. If you’re ever unsure, reach out to your lender or your broker, they can help ensure everything is tracking correctly.

Need a mortgage broker? call us