Imagine tracking your household's grocery spending every month for years. The number bounces around a bit, but it's predictable: somewhere between $14,000 and $15,000 annually. Then one year, halfway through, it suddenly reads $7,683. You'd assume something broke. A mistake. A missing receipt.

That's what happened to Rotorua's food price index in 2014.

From 2010 through 2013, the index held steady. In 2010, it was 14,492. By 2013, it had barely moved: 15,317. The kind of consistency that makes economists comfortable. Then 2014 arrived, and the number crashed to 7,683 (Source: Stats NZ, food-price-index-detailed). Not a gentle decline. A cliff.

This is the lowest Rotorua's food price index has been since 1993. You have to go back more than two decades to find a comparable figure. But here's what makes it strange: there's no obvious explanation in the wider data. Food prices nationally weren't collapsing in 2014. Other regions weren't seeing their indices cut in half.

The most likely answer is a methodology change. Stats NZ periodically updates how it collects and weights price data. Sometimes a basket of goods gets redefined. Sometimes the base year shifts. When that happens, numbers can jump or drop sharply, not because prices changed, but because the measurement did.

But Stats NZ doesn't flag this clearly in the raw dataset. There's no footnote saying "new methodology introduced mid-2014". Just a number that drops by half, sitting there like a typo that nobody fixed.

For anyone trying to understand what Rotorua families actually paid for groceries between 2010 and 2014, this gap is a problem. You can't compare 2013 to 2014 meaningfully. You can't track inflation. You can't answer the most basic question a price index exists to answer: did food get more or less affordable?

This isn't unique to Rotorua. Regional food price data is messy across New Zealand. Different start dates, unexplained gaps, indices that suddenly restart from new baselines. It's the kind of inconsistency that makes headlines impossible and long-term trends unreliable.

The irony is that 2014 matters. It sits right before the housing boom that reshaped New Zealand's cost of living. It's the year before Auckland's median house price broke $700,000. It's the last moment before everything got measurably more expensive. And for Rotorua, the data from that year is half-missing.

Somewhere in a Stats NZ archive, there's probably a technical note explaining the 2014 methodology shift. But for the average New Zealander trying to understand how grocery costs have changed in provincial towns, that explanation might as well not exist. The data just stops making sense, and nobody tells you why.