Illustration of New Zealand one-dollar coins falling through the rain in front of a blurred U.S. Federal Reserve-style building, representing pressure on the Kiwi dollar as U.S. Treasury yields rise.

America’s long-term borrowing costs have climbed to levels not seen since 2004, as investors demand increasingly higher returns to lend money to the U.S. government.

The yield on the 30-year U.S. Treasury rose above 5.44% on Thursday, extending a global bond sell-off that has pushed government borrowing costs sharply higher across major economies.

The move matters well beyond Washington. Treasury yields help set the benchmark for financing throughout the American economy, influencing everything from mortgages and corporate debt to investment decisions and asset valuations.

U.S. mortgage rates are already feeling the pressure. The average rate on a 30-year fixed mortgage reached 7.12% in the week ended September 18, according to the Mortgage Bankers Association, its highest level since May 2024.

Behind the bond sell-off is an increasingly uncomfortable combination of stronger-than-expected U.S. economic growth, persistent inflation pressure, elevated energy prices and concern over the sheer amount of government debt investors are being asked to absorb.

The Federal Reserve has added to the higher-rate backdrop. Last week it raised its benchmark interest-rate range to 3.75% – 4.00%, its first increase in three years. Nearly all Fed policymakers also projected at least one further rate increase before the end of the year.

Long-term Treasury yields behave differently from the Federal Reserve’s overnight policy rate. While short-term yields are closely tied to expectations for Fed decisions, the 30-year yield reflects the return investors require for committing their money to the U.S. government for decades.

That makes the current move particularly significant. Investors are effectively demanding greater compensation for inflation risk, fiscal uncertainty and locking their money away for three decades.

The effects can also travel overseas. U.S. Treasuries sit at the centre of global finance, and persistently higher American yields can attract capital toward U.S. dollar assets while tightening financial conditions elsewhere.

The New Zealand dollar has already been losing ground. NZD/USD has fallen from around US$0.596 in late August to about US$0.567 per NZ$1, a decline of almost 5% in roughly a month.

Higher U.S. Treasury yields could add to that pressure by making American assets more attractive to global investors. The Federal Reserve’s 3.75% – 4.00% policy range also sits above New Zealand’s 2.75% Official Cash Rate, which can reduce the relative appeal of holding Kiwi dollars. If U.S. yields remain elevated or climb further, NZD/USD could face additional downward pressure.