What do you do when half a city's grocery spending disappears in twelve months?

Between 2013 and 2014, Timaru's recorded grocery expenditure fell from $15.4 million to $7.8 million. Not a decline. A collapse. The kind of drop that should have made national headlines, triggered investigations, sent economists scrambling for explanations.

Except nobody noticed. Because it almost certainly didn't happen.

For four consecutive years, Timaru's grocery spending tracked steadily upward: $14.6 million in 2010, $15.3 million in 2011, $15.3 million in 2012, $15.4 million in 2013. Then 2014 arrived and the number halved. (Source: Stats NZ, food-price-index-detailed)

This isn't about Timaru families suddenly eating half as much food. It's about how Stats NZ collects and reports grocery data, and what happens when methodology changes, sample sizes shift, or regional definitions get redrawn without warning.

The timing matters now because, as RNZ reports, soaring bills are forcing New Zealand households to cut spending. When people talk about cost-of-living pressures, they're citing data like this. When economists model grocery inflation, they're working with these numbers. When politicians debate food security, this is the evidence base.

But if a city's grocery spending can apparently halve in a year without explanation, what else in the dataset is unreliable?

Stats NZ's food price index is one of the most-cited economic indicators in New Zealand. It drives policy decisions. It shapes wage negotiations. It influences Reserve Bank interest rate calls. Every household in the country feels its effects.

Yet here's Timaru, with a data discontinuity so dramatic it suggests either a catastrophic economic event nobody reported, or a fundamental problem with how the numbers are gathered.

The 2014 figure of $7.8 million is the lowest on record for Timaru since 1993. To find a comparable number, you have to go back more than two decades. That's not a recession marker. That's a red flag.

This matters beyond Timaru. Regional grocery data underpins national inflation figures. If one city's numbers can swing wildly without explanation, how confident should we be in the aggregate? When the government says food prices rose 3% nationally, are we measuring the same basket in the same places using the same methods?

The Stats NZ dataset offers no footnotes for 2014, no asterisks, no methodological notes. Just a number that defies economic logic, sitting quietly in a spreadsheet, waiting for someone to ask why.

New Zealanders deserve better. Not just better grocery prices, though that matters too. Better data. Data you can trust when you're trying to understand why your weekly shop costs more, or whether your region is doing better or worse than the rest of the country.

Because if the numbers don't make sense, how can the policy decisions built on them possibly work?