GCSB is the Government Communications Security Bureau of New Zealand[1]. There seem to be two bills[2], and the topic has been on HN several times, but never got many points[3].
[1] http://en.wikipedia.org/wiki/Government_Communications_Secur...
It's point (b) that's causing the real controversy, particularly since we're in the 5-eyes network and thus the GCSB are likely data-sharing with everyone else in the network (e.g. NSA, GCHQ.)
In reality the GCSB has been caught illegally spying on NZers, so the new bill is a rush-job to retroactively legalise it, but it's just ridiculously vague, and without even the protections of the US FISA court.
The oversight regime is that warrants are issued by the Prime Minister (the leader of the majority government party) and/or the Director of the GCSB (currently an old school-buddy of the current Prime Minister.) There's an auditor/investigator (one guy) who will get to review things once a year after the fact, although amendments to the bill might make it a panel of three doing the reviewing.
And to really get a grasp on the insanity - watch the Prime Minister likening the system to virus protection [2] - then walking out of the press conference (after saying the Law Society are wrong).
[1] http://www.lawsociety.org.nz/__data/assets/pdf_file/0007/683...
[2] http://www.3news.co.nz/VIDEO-Key-walks-out-of-press-conferen...