It's my understanding that warrants can be issued not just for individuals but "classes" of people—and those warrants are issued not by the judiciary ,but by the PM himself, or his appointed commissioner.

"A "warrant" to further investigate the content of this communication can be issued, not by the Judiciary but by the PM or the Commissioner, and such a warrant can be issued not only for individuals, but for a "class" of people within New Zealand." [1]

[1] http://www.nzherald.co.nz/privacy/news/article.cfm?c_id=546&...