nigelj:
Actually, ethnicity is a good one to poll, while the stats on those subjects are often used by politicians, there is interest in them from other fields. Say health, while uncommon it's not impossible for a medicine to react differently to people of certain ethic descents, an example (that was actually used on MASH) is Primaquine, which can react badly to people of African or Mediterranean descent (see http://en.wikipedia.org/wiki/Primaquine#Adverse_reactions). Now lets say something like that happened again and there was a drug or disease in the market/wild that reacted badly to certain ethnicities (due to say genetics), how does the government know where to deploy the most resources? Ask the Department of Statistics for the most recent census count - okay it might be 4 years out of date, but it might help them realize that say "We don't need to allocate very many resources to Southland for this, because only 1% of the population is [whatever]".
(Just a note on this, I do concede that: The way I recall that the question is asked, is fatally flawed for my example, because say a 2nd generation (living in NZ) person, could identify themselves as NZ European, if say both their parents were of Mediterranean descent and gave birth here, it's a Catch-22 that is likely never able to be solved, but census operations are never 100%, but do still provide the most accurate count/summary of people in NZ to represent them)
Another one is from ethnicity or religion data, a Hospital may decide that because of the large number of [whatever] population/believers that they should add a [insert relevant religious occupation here] or [$language-translator] to their staff/call roster to better cater for the population of the area.
Yes it could be seen as invasive, but I prefer to settle with the belief that there is a good reason/outcome for this, and to be honest, I don't really care if Stats NZ/Govt knows that I'm a NZ European with no religion.
I think you are drawing a fairly long bow with some of that.
The way I understand Stats does Race it isn't based on your preponderance of DNA Anyway. Firstly, it's based on which group you identify with. Secondly, if you are (say) 31/32nds Scottish and only 1/32nd Maori by genetic descent, and you list both, you get classified as Maori by the Stats system. So I doubt they are going to have much luck deciding whether to order a (hypothetical) drug that Scottish people react to and Maori don't. Secondly, as you yourself observe, what people put as their "race" may have no actually connection to a specific ethnicity anyway.
Language translation is a bit or an odd argument. It asks what ethnicity you identify with, not which languages you are fluent in. For instance, I can be fluent in French despite not being French, while many Maori aren't actually fluent in Maori.
And I resent the religion question. That's a private matter, not a matter for the State.
Actually, I somewhat approve of the people who put Jedi in response. From university days, I know that one way of defeating an intrusive survey which you resent is not to comply. Another, especially where non-compliance carries significant risk of inconvenience or penalties, is to give the appearance of compliance but poison the data. If they get (as in a government role I worked in many years ago, where we all got irritated with a very politically correct HR team) a huge percentage of wildly implausible ethnicities then that's pretty much the end of it. [The Europeans, to virtually a man/woman all put Pacific Islander when they made compliance confidential but mandatory, on the basis that NZ is an island in the Pacific and we are NZers. They gave up asking the question after that].
So, if you really object to the Census, one response is just to put nonsense on the form. Roll a dice to pick between options if you need to.
Also just remember that for a supposedly anonymous Census, it isn't any more. Law changes mean that all the forms with your information on them, including your full name and identification etc, are now completely preserved so they can be looked at for all time by "researchers".