For four straight years, Wanganui's annual grocery spend held steady around $15,600. Then 2014 happened, and the number got cut in half overnight.

The official figures show Wanganui residents spending $7,798 on food in 2014, down from $15,641 the year before. That's not a gradual decline. That's not even a sharp correction. That's a statistical cliff.

This matters because these numbers come from Stats NZ's food price index, the same dataset that tracks what every other region in New Zealand pays for groceries. When one region's figures suddenly drop by 50 percent while the rest of the country's costs keep climbing, something fundamental has changed in how that data is being collected or categorised.

Consider what $7,798 actually means. That's $150 a week. For an entire household's groceries. In 2014. While Wellington families were spending double that. While Auckland was higher still. The contrast doesn't make sense unless the methodology shifted.

The trajectory tells the story more clearly than any single year: 2010 saw $14,652, then steady increases through 2013. Four years of predictable growth, the kind you'd expect as food prices tick upward. Then the 2014 figure arrives at less than half the previous year's total, (Source: Stats NZ, food-price-index-detailed).

What likely happened: a change in survey methodology, a shift in how regional data gets aggregated, or a reclassification of what counts as Wanganui versus the broader Manawatū-Whanganui region. Stats NZ periodically updates its collection methods, and regional boundaries sometimes shift in administrative data.

But here's why this matters beyond just one city's accounting: when the numbers change this dramatically, it becomes impossible to track real cost-of-living trends over time. You can't compare 2014 to 2013. You can't measure whether families are actually spending more or less. The data becomes useless for the exact thing it's supposed to measure.

For Wanganui residents trying to understand whether their grocery bills have genuinely changed over the past decade, the official record now has a $7,843 gap right in the middle of it. That's the difference between the 2013 and 2014 figures, and it represents a complete break in the data's continuity.

The 2014 number of $7,798 is the lowest Wanganui has recorded since 1993. But that comparison is meaningless if the 2014 methodology differs from every year before it. You're not comparing like with like. You're comparing two different ways of counting the same thing.

This is how official statistics can tell you everything and nothing at the same time. The numbers are precise. The methodology is presumably sound. But the story they tell has a missing chapter that makes the rest impossible to read.