A Rotorua household checking grocery receipts from 2013 and 2014 wouldn't see their bills cut in half. But the official food price index for the region tells a different story: it dropped from 15,317 to 7,683 in a single year.

This isn't a miracle of price deflation. It's a methodological shift in how Statistics New Zealand measures what we pay for food.

For four consecutive years, Rotorua's index held steady around 15,000. Then it fell off a cliff. The pattern is too clean, too sudden to reflect actual shopping behaviour. Real price movements are messy. They drift up and down with fuel costs, weather, and global supply chains. They don't halve themselves between December and January.

What changed was the basket. Statistics NZ periodically updates which products it tracks and how it weights them to reflect what Kiwis actually buy. When those updates happen, index numbers can jump or plummet even if your actual grocery bill barely moved.

The 2014 figure of 7,683 became the new baseline. It's the lowest Rotorua's index had been since 1993, but only because the measurement system reset. Think of it like switching from miles to kilometres: the distance to the dairy didn't change, but the number you write down did.

This matters because food price indices aren't just academic exercises. They feed into inflation calculations. They inform policy decisions. They shape how politicians talk about cost-of-living pressures. And when the methodology shifts without clear explanation, it becomes nearly impossible for regular people to track whether their grocery bills are actually getting better or worse over time.

The Rotorua data exposes a broader problem: New Zealand's regional food price data is full of these discontinuities. Comparing 2010 figures to 2020 figures often means comparing apples to oranges, sometimes literally. One index might emphasise fresh produce heavily; the next might weight packaged goods differently. The numbers exist, but the story they tell depends entirely on understanding the footnotes.

For a family in Rotorua watching their grocery bills climb year after year, these statistical adjustments don't ease the pressure at the checkout. But they do explain why official data sometimes seems disconnected from lived experience. The index halved. Your shopping bill didn't. That gap between measurement and reality is where trust in economic data goes to die.

Statistics NZ's job is hard: track thousands of products across dozens of regions while accounting for what people actually buy. But when a number drops by half overnight, it needs a footnote the size of a billboard.(Source: Stats NZ, food-price-index-detailed)