There are two possibilities here. Either Whanganui experienced the most dramatic food price collapse in New Zealand history in 2014, or something went catastrophically wrong with how Stats NZ counted the numbers.

The food price index for Whanganui sat at 15,641 in 2013. One year later, it was 7,798. That's not a rounding error. That's not a seasonal adjustment. That's the kind of drop you'd see if half the supermarkets in the region suddenly started giving food away (Source: Stats NZ, food-price-index-detailed).

To put this in perspective: while Whanganui's index supposedly halved, every other region in New Zealand saw gradual increases. Food got more expensive everywhere else. Just not in Whanganui, apparently.

The previous four years tell a different story. From 2010 to 2013, Whanganui tracked steadily upward: 14,652 in 2010, 15,404 in 2011, holding at 15,641 through 2012 and 2013. Normal regional variation. Then the cliff.

Here's what makes this stranger: you have to go back to 1993 to find a comparable figure to that 2014 number. Which would mean Whanganui somehow time-travelled two decades backward in food costs, while the rest of New Zealand kept moving forward.

No newspaper reported on this miracle. No politician took credit. No economist analysed what would have been the most dramatic cost-of-living improvement in modern New Zealand history. Because it almost certainly didn't happen.

What likely did happen: a methodological change, a data collection issue, or a recategorisation that Stats NZ either forgot to flag or buried in technical notes nobody reads. The kind of discontinuity that makes year-on-year comparisons meaningless but still appears in public datasets as if nothing changed.

This matters because people use this data. Researchers cite it. Policy analysts build arguments on it. Journalists write stories from it. And when half the numbers disappear overnight without explanation, every conclusion drawn from them becomes suspect.

The 2014 figure sits there in the official record, looking authoritative, carrying the Stats NZ imprimatur. But put it next to 2013, and the only sensible response is: something's broken.

Either Whanganui had the best year for food affordability in New Zealand history and nobody noticed, or we're all working from a dataset with a hole in it the size of 7,843 index points. One of those explanations makes sense. The other requires magic.