As the government unveils New Zealand's first national infrastructure plan, promising billions for roads and water systems, there's a different kind of infrastructure problem buried in our data. The numbers we use to measure whether Kiwis can afford to eat just broke.

Rotorua's food price index sat at 15,317 in 2013. Then in 2014, it plummeted to 7,683 — a 50% drop that would suggest food suddenly became half as expensive. Spoiler: it didn't. (Source: Stats NZ, food-price-index-detailed)

What happened was Stats NZ changed how they measure food prices. They rebased the index, shifting the reference point and methodology. It's like switching from measuring your height in centimeters to inches — you're the same height, but the number changes. The problem is that anyone looking at this data without context sees a story that doesn't exist.

This matters because New Zealand is in the middle of a genuine cost-of-living crisis. Families need accurate data to understand whether their grocery bills are rising faster than their wages. Policymakers need reliable numbers to decide whether benefit increases are keeping pace with reality. But when the measurement system changes mid-stream, comparing 2013 to 2014 becomes meaningless.

Here's what we know: from 2010 to 2013, Rotorua's food price index climbed steadily from 14,492 to 15,317. That's a real trend — food was getting more expensive. Then the methodology changed, and suddenly we're measuring a different thing with a different number.

The broader issue is that these measurement changes happen across government datasets all the time, often with minimal explanation. You'll be tracking a trend for years, then suddenly the numbers jump or drop for reasons that have nothing to do with the real world. It's statistical infrastructure — the pipes and wires of how we understand our economy — and when it breaks, nobody notices until someone tries to use it.

For Rotorua specifically, the post-2014 data tells us nothing about whether food got cheaper or more expensive compared to the previous four years. We're measuring apples and oranges, literally and figuratively. And because these figures aren't inflation-adjusted, even consistent year-on-year comparisons need careful interpretation. New Zealand's inflation since 2019 alone has been roughly 20-25%, which means a static index number actually represents falling real prices.

The government's new infrastructure plan will spend billions on physical infrastructure. But our data infrastructure — the systems that tell us whether policies are working — needs attention too. When the numbers break, everything built on top of them becomes unreliable.