Picture a Rotorua household planning their weekly shop in early 2014. If they'd been tracking the food price index for their region, they would have seen something bewildering: the number had just dropped from 15,317 to 7,683 in a single year. Not a gentle decline. Not a gradual easing. A collapse by half.

This wasn't because food suddenly became affordable in Rotorua. It wasn't a local price war or a regional subsidy. It was a data discontinuity, the kind that happens when measurement methods change, coverage shifts, or reporting boundaries get redrawn. And it reveals something important about the numbers we use to understand cost-of-living pressure: they're more fragile than we think.

For four consecutive years leading up to 2014, Rotorua's food price index held steady around 15,000. Then it halved overnight. (Source: Stats NZ, food-price-index-detailed)

The 2014 figure of 7,683 is the lowest recorded for Rotorua in 21 years. You'd have to go back to 1993 to find anything comparable. But nobody in Rotorua was celebrating cheaper groceries that year, because the drop wasn't real.

This matters because food price indices aren't abstract economic indicators. They're the foundation of policy decisions, wage negotiations, and benefit indexation. When Treasury calculates how much support families need, when unions argue for pay rises, when councils set living wage rates, they rely on this data. If the measurement wobbles, everything built on top of it wobbles too.

Rotorua's sudden drop isn't unique. Regional food price data is notoriously volatile, partly because the sample sizes are smaller, partly because local market quirks can skew the basket of goods being tracked. A single supermarket closing, a change in what products get surveyed, a shift in how rural areas are weighted: any of these can send the index lurching.

The broader lesson is this: when you see a dramatic one-year change in a long-running data series, especially at a regional level, your first question shouldn't be "what happened?" It should be "did the measurement change?"

For Rotorua residents in 2014, their grocery bills didn't halve. But the index that was supposed to reflect their reality did. And somewhere, someone likely made a decision based on that number, believing it was real.