NZ bank fixed mortgage rates are not set arbitrarily — they are derived from NZ interest rate swap rates, which in turn reflect government bond yields. Understanding this relationship demystifies why fixed mortgage rates sometimes move independently of the RBNZ OCR, and why rates can fall (or rise) weeks before the Reserve Bank acts.
NZ fixed mortgage rates = NZ swap rate (for the equivalent term) + bank margin (~1.5–2.5%). NZ swap rates track NZ government bond yields. Bond yields move daily based on inflation expectations, RBNZ policy signals, global rate movements (especially US Treasuries), and investor risk appetite. When bond yields fall, swap rates fall, and banks can — if they choose — lower their fixed mortgage rates. The lag between a bond yield move and a mortgage rate change is typically 1–4 weeks.
What Are Interest Rate Swaps?
An interest rate swap is a contract between two parties to exchange interest payments — one party pays a fixed rate, the other pays a floating rate, on a notional principal amount.
NZ banks use swaps to manage fixed mortgage risk:
- A bank offers you a 2-year fixed mortgage at 5.89%
- The bank has taken on the risk that rates will move during those 2 years
- To hedge this risk, the bank enters a swap: it pays the 2-year fixed swap rate and receives the floating OCR-linked rate
- Net result: the bank earns the spread between your mortgage rate and the swap rate, regardless of where rates move
NZ interest rate swaps are liquid, actively traded, and price in real time based on market expectations of future interest rates.
The Pricing Formula
For any NZ fixed-rate mortgage term:
$$\text{Mortgage rate} \approx \text{Swap rate (same term)} + \text{Bank margin}$$
Bank margin covers funding costs beyond the swap rate, credit risk, operating costs, and profit. Historically, NZ bank mortgage margins have run 1.5–2.5% above the equivalent swap rate, though this varies by competitive conditions and term.
Example (2-year fixed):
- NZ 2-year swap rate: 4.10%
- Bank margin: 1.65%
- Bank’s 2-year fixed mortgage rate: ~5.75%
If the 2-year swap rate falls to 3.80%, the bank could offer ~5.45% — if it chooses to pass through the full reduction.
What Drives NZ Bond Yields (and Therefore Swap Rates)?
1. RBNZ OCR expectations
The most direct driver of short-term (1–2 year) NZ bond yields. Markets price in the expected path of OCR cuts or hikes over the next 1–3 years. If the market expects the RBNZ to cut the OCR from 3.5% to 2.75% over the next 18 months, 2-year yields will fall to reflect those expected short-term rates.
2. NZ inflation data
Higher-than-expected inflation → RBNZ may keep rates higher for longer → bond yields rise → swap rates rise → fixed mortgage rates rise. Lower inflation → opposite.
3. US Treasury yields
NZ government bond yields are correlated with US Treasuries. Global investors hold both; when US yields fall, NZ bonds become relatively more attractive, prices rise, yields fall. See How the US Fed Affects NZ Mortgage Rates for detail.
4. NZD/USD and offshore funding
NZ banks fund some of their lending in offshore markets (particularly US and European bond markets). The NZD/USD exchange rate and NZ-specific credit spreads affect the all-in cost of offshore funding. A weakening NZD increases the NZD cost of foreign currency funding.
5. Risk appetite and credit spreads
In times of global stress (e.g. March 2020 COVID shock), credit spreads widen — investors demand more yield to hold NZ bank bonds. This pushes up bank funding costs even when swap rates are falling, which limits how much mortgage rates can fall.
The Yield Curve and Fixed Terms
The yield curve shows bond (and swap) yields at different maturities. Its shape affects which fixed mortgage term is cheapest:
Normal yield curve (upward sloping): Longer terms cost more. 5-year fixed rates are higher than 1-year rates.
Inverted yield curve: Short-term rates are higher than long-term rates — meaning 1-year fixed rates are higher than 5-year rates. This happened in NZ in 2022–2024 when the OCR was elevated but markets expected rates to fall. Borrowers who fixed for 5 years at that time often locked in lower rates than short-term fixers.
Flat yield curve: All terms are priced similarly — less differentiation between fixing 1 year vs 5 years.
Practical Implications for Borrowers
Fixed rates can move before the OCR
If the RBNZ signals (or the market anticipates) an OCR cut, bond yields and swap rates fall immediately. Fixed mortgage rates may fall before the OCR cut happens. This is why fixed rates sometimes fall while the floating rate (which directly tracks OCR) stays unchanged.
Competition affects the pass-through
Even if swap rates fall 0.3%, banks may only pass through 0.1–0.2% if they’re protecting margins or if competitor banks haven’t moved. The competitive dynamic among ANZ, ASB, BNZ, Westpac, and Kiwibank affects how quickly falls in swap rates appear in advertised mortgage rates.
Watching the right indicators
For borrowers trying to time when to fix:
| Indicator | What it tells you |
|---|---|
| NZ 2-year swap rate | Best predictor of 2-year fixed mortgage rates in 2–4 weeks |
| NZ 5-year swap rate | Best predictor of 5-year fixed rates |
| US 2-year Treasury yield | Leading indicator for NZ 2-year swap rate |
| RBNZ OCR announcements | Direct driver of floating rates; indirect driver of fixed rates via OCR expectations |
| Bank mortgage rate specials | Banks sometimes offer below-curve specials to hit volume targets |
Where to Track Swap Rates
- RBNZ.govt.nz → Statistics → Interest rates → Interest rate swaps — daily published swap rates
- Interest.co.nz — tracks NZ bank mortgage rates daily and publishes swap rate commentary
- Bloomberg / Reuters — real-time swap rates for market professionals
Frequently Asked Questions
Why did my bank raise fixed rates even though the OCR didn’t change?
Fixed rates track swap rates, not the OCR. If global bond yields rose (perhaps due to a US inflation surprise or rising US Treasury yields), NZ swap rates rose with them, and banks adjusted their fixed mortgage rates accordingly — even without any OCR move.
If swap rates are falling, why isn’t my bank cutting mortgage rates?
Banks lag the swap market — sometimes by weeks. They also consider their existing mortgage book (all the customers currently on fixed rates), competitive positioning, and whether the move in swap rates is likely to be sustained. Sometimes banks are also rebuilding margins and choose not to pass through all of a swap rate fall.
How do I know if fixing now is a good time?
Compare the current fixed rate to the equivalent swap rate. If the mortgage rate is unusually high relative to swap (wide margin), rates may compress later. If the swap rate itself is near recent lows, there may not be much further to fall. No one can reliably predict rates — the best strategy is usually to fix based on your own cashflow certainty needs, not on trying to time the bottom.
Does this apply to floating rates too?
No — floating rates track the RBNZ OCR directly (typically OCR + ~2%). Swap rates and bond yields don’t directly affect floating rates — only the OCR does. This is why floating and fixed rates can move in different directions at the same time.