The real question: can I lower the minimum without falling behind?
An r/AusFinance borrower wanted to refinance to 30 years to reduce the required payment. They asked whether putting spare money in offset could prevent the extra interest normally associated with extending the term.
That question combines two choices: the contractual payment you must make and the amount you actually commit to reducing interest-bearing debt. They can be different.
Start with the same rate so the term effect is visible
Use a fictional $600,000 balance at a constant 6% nominal annual rate, monthly interest and payments, with no fees or opening offset. Compare 20 years remaining with a reset to 30 years. These are modelling assumptions, not current product quotes.
| Item | 20-year term | 30-year term |
|---|---|---|
| Minimum monthly payment | $4,298.59 | $3,597.30 |
| Difference in required payment | Base case | $701.28 less |
| Lifetime interest if only minimums are paid | $431,661 | $695,029 |
The lower payment gives flexibility. If only that payment is made, the loan lasts longer and costs more. The lifetime figures assume the same rate throughout and no offset or extra repayments.
Path 1: spend the $701 difference
Pay $3,597.30 each month and spend the remaining budget. You follow the slower 30-year repayment path. A lower required payment is a cash-flow improvement; it is not, by itself, an interest saving.
If the refinance also changes the rate or fees, model those separately. A rate reduction can help while a term extension works in the opposite direction.
Path 2: pay the same total amount into the loan
Keep paying $4,298.59 despite the lower minimum. With the same balance, rate and payment dates, the debt follows the 20-year path in this simplified example. The 30-year contract provides a lower required minimum, but you are choosing to repay faster.
Confirm that the actual product permits those extra payments and how the lender applies them.
Path 3: pay the lower minimum and retain the difference in offset
Pay $3,597.30 to the mortgage and add the remaining $701.28 to a linked 100% offset on the same monthly date. Leave that money there.
While the offset is below the loan balance, subtracting the growing offset from the mortgage produces the same interest-bearing balance as paying the full $4,298.59 directly into the loan, under these matched assumptions. The mortgage balance is higher, but so is retained cash. Compare debt less offset, not the debt balance alone.
This equivalence depends on the offset being fully effective, matching interest rates and timing, no extra fees, and retaining the money. Daily account transactions, a partial offset, withdrawals or different product pricing change the result. Once the loan is fully offset, further cash does not create negative mortgage interest.
Salary going into offset is not enough on its own
The relevant amount is what remains after bills, spending and repayments. Putting your salary through offset and spending the payment reduction is different from retaining an additional $701.28 each month.
If access to the cash is the reason for choosing this structure, include that flexibility in the decision. It also creates the possibility of spending money that the shorter-term loan would have required you to repay.
How to compare your own refinance
Use the refinance calculator to compare staying, repricing and switching with the same holding period. It includes remaining debt and fees, but holds offset cash constant.
Use the mortgage repayment calculator separately for a growing-offset scenario. Match total monthly cash committed and the timing assumptions. At your chosen end date, compare loan debt, offset cash, cumulative interest and fees.
Also confirm what happens to existing redraw at settlement. It does not automatically become cash in the new offset.
Sources and verification
- Original borrower question: reset to 30 years while using offset. Discussion evidence, not product terms.
- ASIC Moneysmart: switching home loans.
- ASIC Moneysmart: mortgage offset accounts.
- GPS mortgage calculation methodology.
Reviewed 31 October 2026. Calculations are illustrative monthly models, not lender repayment quotes.
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