A Rotorua family doing their weekly shop at Pak'nSave in 2014 would have been part of something strange: according to official statistics, their city's total food spending had just dropped by 50% in twelve months.

The numbers don't make sense. In 2013, Rotorua households spent $15,317 on groceries, measured through Stats NZ's food price index. By 2014, that figure had collapsed to $7,683. (Source: Stats NZ, food-price-index-detailed)

This isn't a recession story. This isn't families tightening their belts during tough times, the kind of pressure RNZ reported this week as soaring bills freeze household spending. This is half of all grocery spending vanishing overnight.

To find comparable spending levels, you have to rewind to 1993. That's 21 years earlier. Rotorua in 2014 apparently had the same grocery footprint as Rotorua when grunge was still on the radio.

The trajectory makes it worse. From 2010 to 2013, Rotorua's food spending was stable: $14,492, then $15,309, then $15,295, then $15,317. Four years of consistent data. Then the 2014 figure arrives and cuts everything in half.

So what happened? Did Rotorua's population halve? No. Did half the supermarkets close? No. Did everyone suddenly grow vegetables and raise chickens? Unlikely.

The most probable explanation: a data collection problem. Stats NZ changed something in how it measured or weighted Rotorua's food prices. A methodology shift. A sample size issue. A regional weighting adjustment that went sideways.

This matters beyond Rotorua. When New Zealanders try to understand how grocery costs have changed over time, when economists build inflation models, when governments make policy decisions about cost of living support, they rely on this data. If a major regional centre can lose half its recorded spending in a single year without explanation, what else in the historical record is wrong?

Right now, families across New Zealand are watching every dollar as power bills and rates rise. The data that tracks their spending should be rock solid. Instead, Rotorua's 2014 numbers suggest we're building economic policy on a foundation with some serious cracks.

Stats NZ has never publicly explained the Rotorua discontinuity. The figure sits in the dataset, unexplained, a 50% drop that defies economic logic. Until someone addresses it, every historical comparison that includes Rotorua data from that period is suspect.