A lump sum payment into your home loan is one of the highest-leverage moves you can make — but the impact varies wildly depending on when in the loan you make it. This calculator shows your specific dollar saving, years off the loan, and the impact of timing.
Why timing matters more than size
Australian home loans are amortising, which means each repayment splits between interest and principal. Early in the loan, most of each payment is interest; late in the loan, most is principal. A lump sum payment hits principal directly and removes the interest that would have been charged on that principal for the remainder of the loan.
That's why a $50,000 lump sum in year 1 of a 25-year loan saves so much more than the same $50,000 in year 20:
- Year 1: $50,000 saves interest on $50,000 for ~24 remaining years
- Year 10: $50,000 saves interest on $50,000 for ~15 remaining years
- Year 20: $50,000 saves interest on $50,000 for ~5 remaining years
At 6%, that's roughly $115k vs $52k vs $14k of saved interest. Same lump sum, very different effect.
When lump sum beats offset
For most owner-occupiers, offset is at least as good as a lump sum because it preserves liquidity. But specific cases where lump sum into the loan wins:
- Fixed-rate loan with no offset — you have nowhere else to put it for equivalent benefit (within the fixed-period cap)
- You'd otherwise spend the cash — locking money out of spending reach is a behavioural win
- End-of-loan tidy-up — when you're 1–2 years from finishing the loan, a final lump sum closes it cleanly
For investors specifically, paying lump sums into an investment loan and then redrawing for personal use can break the tax-deductibility of the loan — an expensive accidental restructure. Investors should default to offset.
When lump sum is the wrong move
A few cases where extra cash should NOT go into the mortgage:
- High-interest debt elsewhere — credit cards at 20%+ get paid first
- No emergency fund — 3–6 months of expenses stays liquid
- Tax-advantaged super contributions — for higher earners, salary-sacrificing into super often beats mortgage repayment after-tax
- Investment opportunity with higher after-tax return — rare but possible (e.g. a business investment, deductible investment loan)
The general rule: a mortgage repayment is a 6% (your loan rate) tax-free, risk-free return. Anything that clearly beats that after tax and risk goes first.
Fixed loan complication
Fixed-rate loans usually cap extra repayments at $10,000–$20,000 per year during the fixed period. Pay above the cap and you trigger break costs — which are calculated based on the difference between your fixed rate and current market rates, applied to the prepaid amount over the remaining fixed period.
If rates have risen since you fixed, break costs are usually small or zero. If rates have fallen, break costs can be substantial — sometimes larger than the interest saving from paying down. Check with your lender for an exact break-cost quote BEFORE making a lump sum payment on a fixed loan.
Hybrid strategy: offset + occasional lump sum
For most owner-occupiers, the optimal pattern is:
- Keep cash in offset by default — preserves liquidity, reduces interest dollar-for-dollar
- Make occasional lump sum payments to "lock in" excess — when offset balance gets large enough that you'd otherwise spend it
- Final-year tidy-up — pay the residual loan in full once it's small enough to clear from savings
This gets you 95% of the offset benefit with the behavioural commitment of locked-down principal.
Frequently asked questions
How much does a $50,000 lump sum save me?
On a $600,000 loan at 6.0% with 25 years remaining, paying $50,000 in year 1 (so making it year 0 of the 25 years) saves roughly $115,000 in interest over the loan and knocks 4 years off. The same $50,000 in year 15 saves only ~$30,000 and knocks off about 1.5 years. Lump sum value drops sharply with time — get it in early.
Should I pay a lump sum or put it in offset?
Mathematically they're equivalent if your lender's redraw is free, instant, and doesn't affect tax deductibility. Otherwise: offset for owner-occupier loans (preserves liquidity), pay-down for fixed loans where redraw isn't possible, and offset for investor loans (paying down then redrawing breaks deductibility). For owner-occupiers with a strong offset facility, the only reason to pay down directly is to permanently 'lock' the money out of your spending reach.
Will my lender let me make a lump sum payment?
On variable rate loans: almost always, unlimited, no fees. On fixed rate loans: usually capped at $10,000–$20,000 per year during the fixed period; payments above the cap incur break costs. After the fixed period ends, full flexibility resumes. Check your loan contract for specific limits.
What if I have a tax refund or work bonus coming?
Same calculation. Time the lump sum to maximise impact: earlier is better. A $20,000 tax refund in year 2 saves dramatically more than the same $20,000 in year 12. If you get a large bonus or refund every year, automating it as an annual lump sum (rather than spending it) is one of the highest-leverage saving moves available to most borrowers.
Should I keep my repayment the same or lower it after a lump sum?
Keep the same repayment if you want to pay the loan off earlier — that's where the big interest saving comes from. Lower the repayment if cashflow is tight and you want to convert the lump sum into ongoing repayment relief. Most borrowers should keep the same repayment because the time and interest saving is much larger than the cashflow benefit, but the choice is yours.
Can I pay a lump sum on a fixed-rate loan?
Limited. Most fixed-rate Australian home loans cap extra repayments at $10,000–$20,000 per year during the fixed period. Payments above the cap trigger break costs that effectively cancel the saving (sometimes worse — break costs can be larger than the interest saving). If you have a large lump sum and a fixed loan, wait for the fixed period to end OR talk to your lender about refinancing.
How does the lump sum interact with my offset balance?
Paying a lump sum into the loan reduces the principal directly. Money sitting in offset already reduces the interest base by the same amount day-by-day. If you have $50k in offset and put another $50k as a lump sum into the loan, your interest charge drops further (now $100k off the principal). But you've lost the liquidity of that second $50k. Most owner-occupiers should max offset before paying lump sums into the loan.
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Last updated: 2 May 2026