Rate changes are the single largest variable in a 30-year loan, and the one borrowers have least control over. Understanding the mechanics makes the outcome far less alarming.
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What your lender changes
When your variable rate rises, more of each repayment is consumed by interest. Lenders generally respond in one of two ways.
Most recalculate your minimum repayment upward so the loan still finishes within its original term. Some instead leave the repayment unchanged and allow the term to extend — which quietly increases the total interest you pay over the life of the loan.
If your repayment did not change after a rate rise, check whether your loan term was extended instead. The cost of that is easy to miss.
Roughly how much a rise costs
As a rule of thumb, each 0.25% increase adds approximately $15 to $16 per month for every $100,000 borrowed, on a 30-year loan.
On a $600,000 loan, a 0.25% rise therefore costs somewhere around $90 to $95 a month. A full 1.00% increase costs roughly four times that.
- $400,000 loan, 0.25% rise: roughly $60–65 more per month.
- $600,000 loan, 0.25% rise: roughly $90–95 more per month.
- $800,000 loan, 0.25% rise: roughly $120–130 more per month.
Why an offset balance protects you
Interest is charged on your balance minus your offset. A larger offset balance means a rate rise applies to a smaller effective debt, so the dollar impact is proportionally reduced.
This is the practical argument for building an offset buffer during periods of lower rates: it is the cheapest insurance available against rises later.
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Fixed rates and the revert trap
A fixed rate protects you for the fixed period only. At the end of that term, the loan reverts — usually to the lender's standard variable rate, which is often higher than what a new customer would be offered.
The sharpest shock is not a gradual rise but the moment a low fixed rate expires. If you have a fixed term ending, model the repayment at the likely revert rate well in advance rather than waiting for the letter.
Building a buffer before you need it
- Model your repayment at two or three percentage points above your current rate and check it remains affordable.
- If it does, consider paying that higher amount now — the surplus reduces principal and builds a genuine cushion.
- Keep an offset balance sufficient to cover several months of repayments.
- Review your rate periodically. Lenders rarely offer their best rate to existing customers unprompted, and asking costs nothing.
Key takeaways
- —A rate rise either increases your repayment or extends your term — check which your lender did.
- —Roughly $15–16 per month per $100,000 borrowed for each 0.25% rise.
- —An offset balance reduces the effective debt a rate rise applies to.
- —The largest shock is usually a fixed rate expiring, not a gradual rise.
General information only. This guide explains how these products generally work. It does not take account of your objectives, financial situation or needs, and is not financial product advice under the Corporations Act 2001 (Cth). Figures are illustrative. Speak to a licensed financial adviser before acting.