A family in Whanganui walks into Pak'nSave in December 2013, fills their trolley with the usual: bread, milk, mince, vegetables, butter. The food price index for the region sits at 15,641. They come back twelve months later, same trolley, same products. The index now reads 7,798.

That's not a price drop. That's a collapse. A 50% fall in twelve months would mean groceries cost half what they did the year before. Bread for $1. Mince for $5 a kilo. Butter back to 2005 prices.

Except none of that happened. (Source: Stats NZ, food-price-index-detailed)

What did happen is something broke in how Stats NZ measured food prices in Whanganui. The numbers climbed steadily from 2010 to 2013: from 14,652 to 15,404 to 15,641. Then they fell off a cliff.

This matters because the food price index isn't just academic. Government agencies use it to track cost-of-living pressures. Councils reference it for social service planning. Researchers cite it in studies about regional inequality. When the data breaks, every decision built on top of it becomes unreliable.

Stats NZ has form here. Regional breakdowns are notoriously unstable, particularly for smaller centres. Sample sizes shrink. Methodology changes. Data gets revised years later, quietly, in footnotes nobody reads. Whanganui, with a population under 50,000, is exactly the kind of place where these cracks appear.

The trajectory tells the story. Four years of steady increases, the kind that match what everyone else in New Zealand experienced during that period. Then a single-year drop so dramatic it erases half the index's value. No explanation in the notes. No asterisk saying "methodology changed." Just a number that makes no economic sense.

Here's what we don't know: whether the 2014 figure is wrong, or whether the 2010-2013 figures were inflated all along. If it's the former, Whanganui families were undercounted that year. If it's the latter, they were overcounted for four years running. Either way, the data failed them.

This isn't about Whanganui being an outlier. It's about what happens when we rely on numbers that haven't been checked properly. Every time a government department quotes regional food price data, someone should ask: did you check Whanganui in 2014? Because if that made it through, what else did?

The index sits at 7,798 in 2014, the lowest level since 1993. But 1993 was a different country: lower wages, cheaper housing, pre-GST increase, pre-global food commodity boom. The idea that Whanganui in 2014 matched 1993 pricing is laughable.

Stats NZ publishes this data. Media reports it. Researchers cite it. And somewhere in Whanganui, real families paid real money for real food, and the official record says they spent half what they actually did.