Breaking a fixed mortgage early: when the maths actually works.

What a break cost actually is

When a bank quotes you a fixed rate, it hedges the cost of funding that rate in the wholesale swap market. If you break the fix early, the bank has to unwind that hedge. The break cost is its compensation for the unwind. The formula is roughly: (your fixed rate minus the bank's current swap rate for the remaining term) × balance × remaining time.

If swap rates have fallen since you fixed, the bank is going to fund the unwind at a lower rate than it is receiving from you, so it loses money. It charges you the difference. If swap rates have risen, the bank can re-lend the money at a higher rate, so the break cost is typically zero or very small.

This is why break costs aren't a penalty in the punitive sense. They are a settlement of the economic position between you and the bank. The Banking Ombudsman has guidance on how the calculation works (Banking Ombudsman NZ).

When the break maths works

Three conditions need to line up. None of them alone is enough; together they make the break a clear decision.

1. Your current rate is materially higher than current carded rates. Not by 10 basis points. By 80 to 200 basis points. If you fixed at 6.99% and the carded 2-year is now 5.79%, you're 120 bp in the money on the new rate.
2. You have meaningful time remaining on the current fix. A break with 3 months left isn't worth the break cost or the energy. A break with 18 to 24 months left has real benefit to capture.
3. A refinance is on the table with cashback. Some banks pay $4,000 to $7,000 per $100,000 of lending to refinance. That cashback partly or fully covers the break cost on a meaningful refinance.

A worked example

Indicative numbers, not a quote. The decision is real; the figures are illustrative.

Mortgage: $720,000 at 6.99% fixed, 22 months remaining.
Available alternative: 2-year fixed at 5.79% at a different bank, with $5,400 cashback ($750 per $100,000 of lending).
Quoted break cost from current bank: $9,800 (as at a specific date; refreshed daily).

The calculation:

• Interest saved at the new rate: 1.20% × $720,000 × 22/12 ≈ $15,840 of interest savings over the next 22 months.
• Cashback received: $5,400.
• Total benefit: ~$21,240.
• Less break cost: $9,800.
• Net benefit: ~$11,440 over 22 months.

That break is mathematically worth doing. Net benefit comfortably exceeds the break cost. Margin of safety is good. The household pockets roughly $11,000 over the remaining term, with the cashback front-loading some of the benefit at settlement.

A worked example where the break does not work

Same client, but now with 8 months left on the fix.

• Interest saved at the new rate: 1.20% × $720,000 × 8/12 ≈ $5,760.
• Cashback received: $5,400.
• Total benefit: ~$11,160.
• Less break cost (lower because less remaining term): ~$3,600.
• Net benefit: ~$7,560.

Still mathematically positive, but the margin is thinner once you account for the application work, the new bank's processing time, and the fact that in 8 months the same household can simply refix into the lower rate without any of this. The right answer is often "wait for the natural expiry and refinance then." There's no break cost on a natural expiry.

What the bank won't tell you about the break quote

• The quote changes daily. The bank's swap rate moves; your break cost moves with it. A $9,800 quote on Monday can be $11,500 on Friday or $8,200. Don't sit on a quote for two weeks.
• The "as at" date matters. Banks quote the break cost as at a specific date, usually 2 or 3 business days out. If you accept after that date, you get a fresh quote.
• You can ask the new bank to factor break cost into cashback. Some refinance offers explicitly negotiate higher cashback when there's a meaningful break cost to cover.

The structural traps

Three structural things kill what otherwise looked like a clean break.

1. Cross-collateralised lending. If your fixed-rate loan is secured against multiple properties, breaking and refinancing typically requires unwinding all of them at the same time. Banks dislike doing this. The application becomes a full re-application.
2. Servicing test rate has tightened. Your original loan was approved at a 7% servicing test. Two years later the test rate is 9%. You qualify for less than you currently borrow. The new bank technically can't take the same loan amount.
3. Property valuation has dropped. If the new bank's valuation comes in lower than your original purchase, the LVR ratio changes, which can move you into a higher-LEM band or push you over the bank's serviceability threshold.

How we run the maths

For any break-or-stay client conversation, we ask the current bank for a written break-cost quote, get a parallel refinance offer from at least two other banks including cashback and new-bank fees, model the interest path on both options over the remaining fixed term and into the next fix, and present the net dollar number for each scenario. The decision belongs to the household. Our job is to make sure they're deciding on numbers, not feel.

(For the related question of whether to refinance even without a break, see our refinance cashback math.)