Still, census data should not be accessible from a public facing web site. That's just amateur hour. You should really assume that anything with a POST form is vulnerable.
As soon as the census closed, the relevant boxes should have been taken offline. The data moved to a "secure" location, and the original boxes wiped and destroyed.
Considering the data that was being collected, I don't think this is overkill.
That's how Australia treats important (Top Secret classified) data. I don't know how classified our census is, it should be treated with a bit of respect.