How Much Does a Lump Sum Repayment Save on Your Mortgage?

Tax refunds, bonuses, and windfalls — here is exactly what each amount saves on a typical Australian home loan.

A lump sum payment applied directly to your mortgage — whether it is a tax refund, work bonus, inheritance, or the proceeds from selling an asset — can save a surprising amount of interest and meaningfully shorten your loan term. A single $20,000 payment made in year one of a $600,000 loan can save over $91,000 in total interest. Here is why, and how to calculate the saving for your own loan.

Lump Sum Savings — $600,000 Loan at 6.00%, 30-Year Term

The table shows how much interest is saved by a single lump sum payment made either at year 1 (start of the loan) or year 5.

$600,000 loan at 6.00% p.a., 30-year term. Lump sum applied as a single extra payment in the stated year, minimum repayments continue unchanged. Interest savings are approximate.

Why Lump Sums Save So Much More Than Their Face Value

The numbers above might seem surprising — how does a $10,000 payment save nearly $48,000 in interest? The answer is that you are not just saving interest on $10,000 for one month. You are saving interest on $10,000 for every remaining month of the loan — because that principal is gone from the balance permanently.

On a $600,000 loan at 6%, the monthly interest on $10,000 of principal is approximately $50. Over the remaining 29 years of the loan — roughly 348 months — that is around $17,400 in direct interest. But because the balance is lower, each future repayment also pays off principal faster, which in turn reduces future interest. The cascading effect of this compounding is what produces the $47,704 saving.

In short: a lump sum does not just eliminate interest on that amount once — it eliminates it for the remaining life of the loan.

Why Timing Matters So Much

Compare $10,000 applied at year 1 versus year 5 in the table above. The year 1 payment saves $47,704; the year 5 payment saves $33,217 — about 30% less for waiting four years. And the later you pay, the smaller the saving becomes.

This is not because the maths changes — it is because there are fewer months remaining for the compounding saving to accumulate. If you have a windfall today, the best time to apply it to your mortgage is immediately. Every month you wait costs you roughly 0.5% of the lump sum in foregone interest savings (at 6% per annum).

Common Sources of Lump Sum Repayments

Tax refunds

The average Australian tax refund is around $2,400–$3,000. Applied directly to the mortgage each year, a $2,400 refund at 6% saves roughly $10,000–$12,000 in interest over a 30-year loan — and that saving grows each year if you make it a habit. Use the calculator to add a recurring annual lump sum to model this.

Work bonuses

A $10,000–$20,000 annual bonus applied to your mortgage each year is one of the most powerful uses of a performance incentive. Over five years, consistent bonuses applied to the loan can save well over $100,000 in interest — significantly outperforming leaving the money in a high-interest savings account, particularly after accounting for tax on savings interest.

Inheritances and windfalls

A $50,000 windfall applied in the first year of a $600,000 mortgage saves over $200,000 in interest — a four-to-one return. If the windfall arrives later in the loan term the saving is smaller, but still significant. Model it with the specific date you expect to receive the funds for an accurate figure.

Selling an asset

Proceeds from selling a car, shares, or a rental property (after any capital gains tax) can make a meaningful dent in your mortgage. Even a $15,000 asset sale applied to a $600,000 loan in year one saves nearly $70,000 in interest.

Fixed Rate Loans — Check Before You Pay

Variable rate mortgages in Australia generally allow unlimited extra repayments with no penalty. Fixed rate mortgages typically cap extra repayments at $10,000–$20,000 per year during the fixed period. If you exceed this cap, break costs may apply — and these can be significant if rates have fallen since you fixed.

Always call your lender before making a large lump sum payment on a fixed rate loan to confirm the cap and any associated costs.

How to Model a Lump Sum in the Calculator

1. Enter your loan details in Section 1 — Loan Details.
2. Open Section 3 — Ad-Hoc Extra Repayments and click Add Row.
3. Enter the date you expect to make the payment and the dollar amount.
4. Add multiple rows if you plan to make more than one lump sum (e.g., your tax refund each July).
5. Click Calculate My Paydown Plan to see the exact interest saved and your new payoff date.