A population expert has accused Statistics New Zealand of undermining the latest Census by creating a misleading impression of New Zealand's ethnic make-up.
Professor Paul Spoonley says the introduction of the new category to count the number of people who declare themselves a "New Zealander" has distorted the true picture.
He says the change to ethnicity in last year's $25 million Census means its data on the size of different ethnic groups cannot be compared with earlier Census counts.
"Statistics New Zealand have thrown a curve-ball into the ethnicity statistics," said Professor Spoonley, Massey University's Auckland regional director of humanities and social sciences.
"We've got no way of comparing the 1996 or 2001 ethnicity statistics with 2006 because we've got this new category which doesn't equate with any previous category."
Until last year's Census, the department categorised people who wrote terms such as "New Zealander" or "Kiwi" in the five-yearly Census as NZ Europeans.
Last year, however, it bowed to public opinion and counted them in a separate category, then lumped them in with the "Other" ethnicities. The number writing "New Zealander" or the like rose more than four-fold, to 429,429, or 11.1 per cent of the population.
This shift appears largely responsible for the sharp decline in the percentage of Europeans in the Census - from 80 per cent in 2001 to 67.6 per cent last year.
Professor Spoonley said: "We don't know how those people answered the ethnicity question in the 2001 Census; we can guess that many of those who said they were New Zealanders were Pakeha or European, but we have no way of knowing."
Otago University cancer researcher Dr Brian Cox said the change could make it more difficult to use Census data in health research and skew data on disease trends if hospitals recorded ethnicity differently from the Census.
But Statistics NZ's social conditions manager, Paul Brown, defended the change. He said health agencies would record ethnicity in the same way, avoiding the issue Dr Cox had raised.
He said the change was simply to cater for an emerging group. "They've said this is their ethnicity. It's not for us to tell people what their ethnicity is."
The ethnicity question was about how people saw themselves, not their ancestry.