This is an educational explainer about how a category of government bond generally works. It is not investment advice, and it does not describe any specific current event, company, or security.
A government bond paying a coupon of just 2 or 3 percent can, in an inflationary year, end up delivering more total cash to its holder than a bond from the same issuer paying 5 percent, without the coupon rate itself ever moving. The trick isn’t in the interest payment. It’s in something happening quietly underneath it: the face value of the bond is not fixed. New Zealand’s inflation-indexed bonds, issued by the New Zealand Debt Management Office (NZDMO), are built around exactly this feature, and understanding how it works explains why two government securities from the same borrower can behave very differently once inflation moves.
How the CPI adjusts the bond’s principal
A conventional government bond has a face value that stays constant for its entire life, typically NZ$100, with a fixed coupon paid on that amount. An inflation-indexed bond (IIB) works differently. Its principal is periodically recalculated using changes in the Consumers Price Index (CPI), the economy-wide measure of price changes published quarterly by Stats NZ. As the CPI rises, the bond’s inflation-adjusted principal, sometimes called its “indexed” or “capital” value, rises with it. If prices were to fall, the adjustment works in reverse.
Because the coupon rate set at issuance is applied to this adjusted principal rather than to the original face value, the actual dollar coupon paid out changes over time even though the percentage rate never does. The same logic applies at maturity: the amount repaid to the holder is based on the inflation-adjusted principal, not the original issue amount, so compensation for cumulative inflation is built into both the running coupon payments and the final redemption.
Why this produces a real yield instead of a nominal one
This mechanism is what allows the coupon rate on an IIB to be described as a real yield. A standard fixed-rate bond promises a fixed number of dollars regardless of what happens to prices afterward, which means its yield is nominal: the price investors are willing to pay for it already has to bake in their expectations for future inflation over the life of the bond. An IIB instead handles inflation separately, through the automatic principal adjustment, so the quoted coupon can represent, at least in theory, the return investors require simply for lending money and taking on risk, with the inflation component stripped out and settled later through the CPI mechanism.
Because these are two structurally different promises, comparing a nominal yield directly with a real yield can be misleading. Market participants instead often look at the gap between the nominal yield on a conventional government bond and the real yield on an IIB of similar maturity, sometimes called the breakeven inflation rate, as a rough gauge of what the market expects inflation to average over that period.
What happens when actual inflation differs from expectations
The practical effect of this design shows up when realized CPI inflation ends up higher or lower than what was priced in when the bond was issued or purchased. If inflation runs hotter than expected, the bond’s principal grows faster than anticipated, lifting the dollar value of both coupon payments and the eventual redemption amount, an outcome designed to preserve purchasing power rather than to generate an unusually large gain. If inflation comes in lower than expected, or turns negative, the adjustment works against the holder in nominal terms, though many IIB structures include a floor ensuring the redemption value at maturity does not drop below the bond’s original face value.
This is the essential difference between the two instruments. A conventional fixed-rate bond defines its return in dollars from day one. An inflation-indexed bond defines its return in terms of purchasing power, with the actual dollar figures filled in only after the CPI data confirms what inflation turned out to be.
