I don't believe there is any justification for census prosecutions. I can think of only two reasons for them:

1. To obtain the withheld data from the non-respondent.
2. To deter future non-respondents.

The first doesn't really make sense; as I read the Statistics Act (1975), people are only required to supply:

...name and address, sex, age, and ethnic origin...

They already know Nik Haden's name, address and sex. So the only gain in data would be his age and ethnic origin. And as Craig R has shown people often don't put their ethnic origin (or sometimes put something meaningless).

Realistically, prosecuting non-respondents does not add to the data, or its quality.

As for the second point, this is largely a matter of prediction so no-one can really say what will happen. But if anyone is of the opinion that census prosecutions are snapping the rest of the population into line, I would ask them what they think of official SNZ policy to be "reactive" to prosecution cover in the media. Surely, if you're going to make an example of someone/ some group, you would want people to know about it.

The vast majority of people fill out their Census forms because they believe it's a good idea, not because they're afraid of going to court.

In my view, those two reasons are impractical. If anyone can think of another justification I'd be interested in hearing it.

Also, I haven't seen any comment mention the costs of prosecution. I can't be certain, but with the costs of the prosecution staff salaries and Crown Law, etc. for 72 court cases -- I estimate SNZ has probably spent well over $100,000. Nik Haden's fine could be up to $1,500.

And just as prosecutions have the potential to scare people into compliance, it has just as much potential to persuade someone into refusing as an act of defiance. Possibly, a "soft-refusal", i.e. just hiding from the form collector which is just as bad for the dataset but doesn't attract attention like a census-form-BBQ.

And Russell: if you value census data, my advice is to worry less about whether 1 of ~92,000 miscounted people are getting their punishment, and ask SNZ if five years and a ~$70 million budget is enough time and money to get the results that are of the standard you're expecting.